feat: add solutions to lc/lcof problems

Author: yanglbme

lcof/面试题64. 求1+2+…+n/README.md

@@ -60,6 +60,21 @@ var sumNums = function (n) {
 };
 ```
 
+### **Go**
+
+```go
+func sumNums(n int) int {
+	s := 0
+	var sum func(int) bool
+	sum = func(n int) bool {
+		s += n
+		return n > 0 && sum(n-1)
+	}
+	sum(n)
+	return s
+}
+```
+
 ### **...**
 
 ```

lcof/面试题64. 求1+2+…+n/Solution.go

@@ -0,0 +1,10 @@
+func sumNums(n int) int {
+	s := 0
+	var sum func(int) bool
+	sum = func(n int) bool {
+		s += n
+		return n > 0 && sum(n-1)
+	}
+	sum(n)
+	return s
+}

solution/1900-1999/1913.Maximum Product Difference Between Two Pairs/README.md

@@ -90,6 +90,29 @@ var maxProductDifference = function(nums) {
 };
 ```
 
+### **C++**
+
+```cpp
+class Solution {
+public:
+    int maxProductDifference(vector<int>& nums) {
+        sort(nums.begin(), nums.end());
+        int n = nums.size();
+        return nums[n - 1] * nums[n - 2] - nums[0] * nums[1];
+    }
+};
+```
+
+### **Go**
+
+```go
+func maxProductDifference(nums []int) int {
+	sort.Ints(nums)
+	n := len(nums)
+	return nums[n-1]*nums[n-2] - nums[0]*nums[1]
+}
+```
+
 ### **...**
 
 ```

solution/1900-1999/1913.Maximum Product Difference Between Two Pairs/README_EN.md

@@ -111,6 +111,29 @@ var maxProductDifference = function(nums) {
 };
 ```
 
+### **C++**
+
+```cpp
+class Solution {
+public:
+    int maxProductDifference(vector<int>& nums) {
+        sort(nums.begin(), nums.end());
+        int n = nums.size();
+        return nums[n - 1] * nums[n - 2] - nums[0] * nums[1];
+    }
+};
+```
+
+### **Go**
+
+```go
+func maxProductDifference(nums []int) int {
+	sort.Ints(nums)
+	n := len(nums)
+	return nums[n-1]*nums[n-2] - nums[0]*nums[1]
+}
+```
+
 ### **...**
 
 ```

solution/1900-1999/1913.Maximum Product Difference Between Two Pairs/Solution.cpp

@@ -0,0 +1,8 @@
+class Solution {
+public:
+    int maxProductDifference(vector<int>& nums) {
+        sort(nums.begin(), nums.end());
+        int n = nums.size();
+        return nums[n - 1] * nums[n - 2] - nums[0] * nums[1];
+    }
+};

solution/1900-1999/1913.Maximum Product Difference Between Two Pairs/Solution.go

@@ -0,0 +1,5 @@
+func maxProductDifference(nums []int) int {
+	sort.Ints(nums)
+	n := len(nums)
+	return nums[n-1]*nums[n-2] - nums[0]*nums[1]
+}