[leetcode 5]longest palindromic substring

1 题目：

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.

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 String

2 思路

开始我想了一种思路，对于那个特例输入：

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabcaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

时间超了，我也没想出其他的办法。

后来看这个帖子：https://leetcode.com/discuss/17622/optimized-n-square-solution-in-java

它是采用了贪心法。里面解释也很详细，不再说了。

自己动手敲了一遍，实现了。

 

3 代码

 

最开始我的代码，时间超了

public class Solution {
    public String longestPalindrome(String s){
        int end = s.length() - 1;
        int length = end + 1;
        String longestSubString = "";
        for (int i = 0; i < length; i++) {
            int start = i;
            int rear = end;
            int temp = rear;
            while( start < rear ){
                if (s.charAt(start) == s.charAt(rear)) {
                    start++;
                    rear--;
                }else {
                    start = i;
                    temp--;
                    rear = temp;
                }
                
                if (start >= rear) {//结束了
                    if (longestSubString.length() < s.substring(i, temp + 1).length()) {
                        longestSubString = s.substring(i, temp+1);
                    }
                    if(longestSubString.length() == s.substring(i,end+1).length()){
                        return longestSubString;
                    }
                }
            
            }
        }
        
        return longestSubString;
    }
}

看懂别人代码后，自己实现的：

public class Solution {
    public String longestPalindrome(String s){
        char[] chars = s.toCharArray();
        int len = s.length();
        
        while(len >= 0){
            for(int i = 0; i + len - 1 < chars.length; i++){
                int right = i;
                int left = i + len - 1;
                
                boolean isPalin = true;
                while(right <= left){
                    if(chars[right] == chars[left]){
                        right++;
                        left--;
                        continue;
                    }else{
                        isPalin = false;
                        break;
                    }
                }
                if(isPalin){
                    return s.substring(i, i + len);
                }
            }
            len--;
        }
        
        return "";
    }
}