第八周：[Leetcode]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”

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dp方法：布尔数组dp[i][j]代表i到j的子字符串是否是回文串，状态转移方程为：
若s[i]==s[j],dp[i][j] = dp[i+1][j-1] || j - i <= 2(长度小于2，”a”或”aa”这种情况)
其他情况都为false。

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class Solution {public:    string longestPalindrome(string s) {        if(s.length() <= 1)            return s;            vector<vector<bool>> dp(s.length(),vector<bool>(s.length()));    int start = 0,longest = 0,length;    for(int i = s.length() - 1;i >= 0;i--){        for(int j = i;j < s.length();j++){            if(s[i] == s[j] && (j - i <= 2 || dp[i + 1][j - 1])){                dp[i][j] = true;                length = j - i + 1;                if(length > longest){                    start = i;                    longest = length;                }            }        }    }    return s.substr(start,longest);    }};