Add solution 005 [Java]

Author: yanglbme

README.md

@@ -47,6 +47,7 @@ Complete [solutions](https://github.com/doocs/leetcode/tree/master/solution) to
 | # | Title | Tags |
 |---|---|---|
 | 002 | [Add Two Numbers](https://github.com/doocs/leetcode/tree/master/solution/002.Add%20Two%20Numbers) | `Linked List`, `Math` |
+| 005 | [Longest Palindromic Substring](https://github.com/doocs/leetcode/tree/master/solution/005.Longest%20Palindromic%20Substring) | `String`, `Dynamic Programming` |
 | 015 | [3Sum](https://github.com/doocs/leetcode/tree/master/solution/015.3Sum) | `Array`, `Two Pointers` |
 | 019 | [Remove Nth Node From End of List](https://github.com/doocs/leetcode/tree/master/solution/019.Remove%20Nth%20Node%20From%20End%20of%20List) | `Linked List`, `Two Pointers` |
 | 024 | [Swap Nodes in Pairs](https://github.com/doocs/leetcode/tree/master/solution/024.Swap%20Nodes%20in%20Pairs) | `Linked List` |

solution/005.Longest Palindromic Substring/README.md

@@ -0,0 +1,60 @@
+## 最长回文子串
+### 题目描述
+
+给定一个字符串 **s**，找到 **s** 中最长的回文子串。你可以假设 **s** 的最大长度为1000。
+
+**示例 1：**
+```
+输入: "babad"
+输出: "bab"
+注意: "aba"也是一个有效答案。
+```
+
+**示例 2：**
+```
+输入: "cbbd"
+输出: "bb"
+```
+
+### 解法
+- 解法1
+
+利用动态规划，二维数组 `res` 存储 `[j, i]` 区间是否为回文串。动态规划递推式：
+
+`res[j][i] = res[j + 1][i - 1] && chars[j] == chars[i]`
+
+此方法时间和空间复杂度均为 `O(n²)`。
+
+```java
+class Solution {
+    public String longestPalindrome(String s) {
+        if (s == null || s.length() < 1) {
+            return "";
+        }
+        String str = "";
+        char[] chars = s.toCharArray();
+        int len = chars.length;
+        boolean[][] res = new boolean[len][len];
+        int start = 0;
+        int max = 1;
+        for (int i = 0; i < len; ++i) {
+            for (int j = 0; j <= i; ++j) {
+                
+                res[j][i] = i - j < 2
+                    ? chars[j] == chars[i]
+                    : res[j + 1][i - 1] && chars[j] == chars[i];
+                
+                if (res[j][i] && max < i - j + 1) {
+                    max = i - j + 1;
+                    start = j;
+                }
+                    
+            }
+        }
+        
+        return s.substring(start, start + max);
+        
+    }
+}
+```
+

solution/005.Longest Palindromic Substring/Solution.java

@@ -0,0 +1,28 @@
+class Solution {
+    public String longestPalindrome(String s) {
+        if (s == null || s.length() < 1) {
+            return "";
+        }
+        String str = "";
+        char[] chars = s.toCharArray();
+        int len = chars.length;
+        boolean[][] res = new boolean[len][len];
+        int start = 0;
+        int max = 1;
+        for (int i = 0; i < len; ++i) {
+            for (int j = 0; j <= i; ++j) {
+
+                res[j][i] = i - j < 2 ? chars[j] == chars[i] : res[j + 1][i - 1] && chars[j] == chars[i];
+
+                if (res[j][i] && max < i - j + 1) {
+                    max = i - j + 1;
+                    start = j;
+                }
+
+            }
+        }
+
+        return s.substring(start, start + max);
+
+    }
+}