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 1:

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

Example 2:

Input: "cbbd"
Output: "bb"

 

 

p[i][j]表示字符序列i到j间的回文状态

if p[i+1][j-1]==True && s[i] == s[j]:

　　p[i][j] = True

 

C++

class Solution {
public:
    string longestPalindrome(string s) {
        bool p[1002][1002];
        memset(p, false, sizeof(p));
        int n = s.length();
        for(int i=0; i<n; i++)
            p[i][i] = true;
        int maxgap = 0;
        for(int i=n-1; i>=0; i--) {
            for(int j=i+1; j<n; j++) {
                if((i+1 > j-1 || p[i+1][j-1]) && s[i] == s[j]) {
                    p[i][j] = true;
                    maxgap = max(maxgap, j-i+1);
                }
            }
        }
        int i, j=n-1;
        cout<< maxgap<<endl;
        for(i=0; i<n; i++, j=n-1) {
            while(j>i && p[i][j]==false)
                j--;
            if(j>i && j-i+1 == maxgap) break; // found the final result
        }
//        cout<< i<<" " <<j<<endl;
        if(i >= j)
            return s.substr(0, 1);
        else
            return s.substr(i, j-i+1);
    }
};