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feat: add solutions to lc problem: No.1711
No.1711.Count Good Meals
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‎solution/1700-1799/1711.Count Good Meals/README.md

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## 解法
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**方法一：哈希表**
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用最暴力的方法枚举每对元素肯定会超时，可以用哈希表优化对**之前元素出现次数**的查询。
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**方法二：枚举二的幂**
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用哈希表 `cnt` 记录数组中每个元素出现的次数。
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枚举二的幂次方作为两数之和 `s`，然后枚举其中一个数 `a`，判断 `s-a` 是否出现在哈希表 `cnt` 中。若出现，判断 `a` 与 `b` 是否相等，是则答案累加 `cnt[a] * (cnt[a]-1)`，否则答案累加 `cnt[a] * cnt[b]`。
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由于每个 `a`，`b` 会重复枚举，因此最后答案需要除以 `2`。注意取模操作。
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时间复杂度 $O(n\log C)$，其中 $n$ 是数组 `deliciousness` 的长度，而 $C$ 是元素的上限，对于本题，上限 $C=2^{20}$。
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<!-- 这里可写通用的实现逻辑 -->
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<!-- tabs:start -->
@@ -69,6 +81,23 @@ class Solution:
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return pairs
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```
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```python
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class Solution:
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def countPairs(self, deliciousness: List[int]) -> int:
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cnt = Counter(deliciousness)
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ans = 0
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mod = 10**9 + 7
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for i in range(22):
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s = 1 << i
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for a, m in cnt.items():
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if (b := s - a) in cnt:
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if a == b:
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ans += m * (m - 1)
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else:
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ans += m * cnt[b]
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return (ans >> 1) % mod
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```
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### **Java**
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<!-- 这里可写当前语言的特殊实现逻辑 -->
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}
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```
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```java
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class Solution {
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private static final int MOD = (int) 1e9 + 7;
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public int countPairs(int[] deliciousness) {
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Map<Integer, Integer> cnt = new HashMap<>();
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for (int v : deliciousness) {
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cnt.put(v, cnt.getOrDefault(v, 0) + 1);
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}
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long ans = 0;
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for (int i = 0; i < 22; ++i) {
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int s = 1 << i;
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for (var x : cnt.entrySet()) {
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int a = x.getKey(), m = x.getValue();
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int b = s - a;
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if (!cnt.containsKey(b)) {
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continue;
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}
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if (a == b) {
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ans += (long) m * (m - 1);
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} else {
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ans += (long) m * cnt.get(b);
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}
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}
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}
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ans >>= 1;
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return (int) (ans % MOD);
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}
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}
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```
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### **Go**
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```go
@@ -124,6 +184,57 @@ func max(x, y int) int {
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}
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```
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```go
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func countPairs(deliciousness []int) int {
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cnt := map[int]int{}
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for _, v := range deliciousness {
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cnt[v]++
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}
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ans := 0
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mod := int(1e9) + 7
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for i := 0; i < 22; i++ {
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s := 1 << i
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for a, m := range cnt {
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b := s - a
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if n, ok := cnt[b]; ok {
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if a == b {
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ans += m * (m - 1)
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} else {
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ans += m * n
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}
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}
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}
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}
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ans >>= 1
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return ans % mod
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}
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```
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### **C++**
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```cpp
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class Solution {
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public:
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int countPairs(vector<int>& deliciousness) {
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unordered_map<int, int> cnt;
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for (int v : deliciousness) ++cnt[v];
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long long ans = 0;
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for (int i = 0; i < 22; ++i) {
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int s = 1 << i;
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for (auto& [a, m] : cnt) {
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int b = s - a;
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if (!cnt.count(b)) continue;
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if (a == b) ans += 1ll * m * (m - 1);
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else ans += 1ll * m * cnt[b];
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}
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}
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ans >>= 1;
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int mod = 1e9 + 7;
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return (int) (ans % mod);
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}
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};
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```
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### **...**
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```

‎solution/1700-1799/1711.Count Good Meals/README_EN.md

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return pairs
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```
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```python
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class Solution:
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def countPairs(self, deliciousness: List[int]) -> int:
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cnt = Counter(deliciousness)
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ans = 0
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mod = 10**9 + 7
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for i in range(22):
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s = 1 << i
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for a, m in cnt.items():
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if (b := s - a) in cnt:
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if a == b:
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ans += m * (m - 1)
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else:
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ans += m * cnt[b]
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return (ans >> 1) % mod
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```
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### **Java**
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```java
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}
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```
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```java
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class Solution {
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private static final int MOD = (int) 1e9 + 7;
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public int countPairs(int[] deliciousness) {
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Map<Integer, Integer> cnt = new HashMap<>();
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for (int v : deliciousness) {
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cnt.put(v, cnt.getOrDefault(v, 0) + 1);
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}
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long ans = 0;
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for (int i = 0; i < 22; ++i) {
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int s = 1 << i;
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for (var x : cnt.entrySet()) {
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int a = x.getKey(), m = x.getValue();
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int b = s - a;
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if (!cnt.containsKey(b)) {
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continue;
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}
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if (a == b) {
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ans += (long) m * (m - 1);
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} else {
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ans += (long) m * cnt.get(b);
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}
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}
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}
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ans >>= 1;
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return (int) (ans % MOD);
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}
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}
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```
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### **Go**
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```go
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}
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```
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```go
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func countPairs(deliciousness []int) int {
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cnt := map[int]int{}
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for _, v := range deliciousness {
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cnt[v]++
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}
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ans := 0
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mod := int(1e9) + 7
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for i := 0; i < 22; i++ {
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s := 1 << i
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for a, m := range cnt {
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b := s - a
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if n, ok := cnt[b]; ok {
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if a == b {
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ans += m * (m - 1)
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} else {
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ans += m * n
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}
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}
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}
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}
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ans >>= 1
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return ans % mod
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}
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```
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### **C++**
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```cpp
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class Solution {
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public:
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int countPairs(vector<int>& deliciousness) {
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unordered_map<int, int> cnt;
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for (int v : deliciousness) ++cnt[v];
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long long ans = 0;
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for (int i = 0; i < 22; ++i) {
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int s = 1 << i;
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for (auto& [a, m] : cnt) {
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int b = s - a;
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if (!cnt.count(b)) continue;
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if (a == b) ans += 1ll * m * (m - 1);
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else ans += 1ll * m * cnt[b];
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}
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}
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ans >>= 1;
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int mod = 1e9 + 7;
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return (int) (ans % mod);
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}
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};
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```
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### **...**
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```

‎solution/1700-1799/1711.Count Good Meals/Solution.cpp

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class Solution {
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public:
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int countPairs(vector<int>& deliciousness) {
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unordered_map<int, int> cnt;
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for (int v : deliciousness) ++cnt[v];
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long long ans = 0;
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for (int i = 0; i < 22; ++i) {
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int s = 1 << i;
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for (auto& [a, m] : cnt) {
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int b = s - a;
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if (!cnt.count(b)) continue;
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if (a == b) ans += 1ll * m * (m - 1);
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else ans += 1ll * m * cnt[b];
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}
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}
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ans >>= 1;
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int mod = 1e9 + 7;
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return (int) (ans % mod);
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}
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};

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