leetcode: Longest Palindromic Substring

http://oj.leetcode.com/problems/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.

思路：

这题有牛B的算法，大家可以自己Google。这里用一种比较简单的算法，虽然不够快但是也能过OJ。如果一个字符串已经是回文字符串，向左右扩展，如果两个新增加的字母相同，那么扩展后的字符串也是回文。奇数偶数情况分别处理即可。

class Solution {
public:
    int extend(const string &s, int start, int end) {
        int len = s.length(), steps = 0;
        
        while ((start >= 0) && (end < len)) {
            if (s[start] == s[end]) {
                --start;
                ++end;
                ++steps;
            }
            else {
                break;
            }
        }
        
        return steps;
    }
    
    string longestPalindrome(string s) {
        int start = 0, max = 0;
        
        for (int i = 0; i < s.length(); ++i) {
            int steps, sub_len;
            
            steps = extend(s, i, i + 1);
            sub_len = steps * 2;
            
            if (sub_len > max) {
                start = i - steps + 1;
                max = sub_len;
            }
            
            steps = extend(s, i - 1, i + 1);
            sub_len = steps * 2 + 1;
            
            if (sub_len > max) {
                start = i - steps;
                max = sub_len;
            }
        }
        
        return s.substr(start, max);
    }
};