Leetcode:Longest Palindromic Substring 最长回文子串

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, and there exists one unique longest palindromic substring.

 

 

解题分析：

设f[i][j]表示[i,j]之间的最长回文子串，递推方程如下：

class Solution {

public:

    string longestPalindrome(string s) {

        if (s.size() == 0) return "";

        const int strLen = s.size();

        bool flag[strLen][strLen];

        memset(flag, false, sizeof(flag));

        //vector<vector<bool>> flag(strLen, vector<bool>(strLen, false));

        int maxLen = 1;

        int start = 0;

        

        for (int i = strLen - 1; i >= 0; --i) {

            flag[i][i] = true;

            for (int j = i + 1; j < strLen; ++j) {

                if (j == i + 1 && s.at(i) == s.at(j)) flag[i][j] = true;

                if (j >  i + 1 && s.at(i) == s.at(j) && flag[i+1][j-1] == true) flag[i][j] = true;

                if (flag[i][j] == true && maxLen < j - i + 1) {

                    maxLen = j - i + 1;

                    start = i;

                }

            }

        }

        return s.substr(start, maxLen);

    }

};

注意：

因为f[i,j] 需要使用到 f[i+1, j-1]，所以必须首先计算 f[i+1, j-1], 然后计算 f[i,j] 这样游标i必须从大到小，游标j必须从小到大