5. Longest Palindromic Substring

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example:

Input: "babad"Output: "bab"Note: "aba" is also a valid answer.

Example:

Input: "cbbd"Output: "bb"

题意：最长回文字符串，用一个二维矩阵来表示，最初从对角线（左上到右下，代表单个字符）开始，然后依次往上进行对角线（两个字符，多个字符）处理。

class Solution {public:    string longestPalindrome(string s) {        int len = s.length();        int maxLen = 0;        int longestBegin = 0;        vector<vector<bool> > vec(len, vector<bool>(len, false));        for(int i = 0; i < len; ++i){//单个字符            vec[i][i] = true;            maxLen = 1;            longestBegin = i;        }        for(int i = 0; i < len - 1; ++i){//相邻两个相等的字符            if(s[i] == s[i + 1]){                maxLen = 2;                longestBegin = i;                vec[i][i + 1] = true;            }        }        for(int l = 3; l <= len; ++l){            for(int i = 0; i < len - l + 1; ++i){                int j = i + l - 1;                if(s[i] == s[j] && vec[i + 1][j - 1]){                    maxLen = l;                    longestBegin = i;                    vec[i][j] = true;                }            }        }        return s.substr(longestBegin, maxLen);    }};