[leetcode-214]Shortest Palindrome(java)

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问题描述：
Given a string S, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.

For example:

Given “aacecaaa”, return “aaacecaaa”.

Given “abcd”, return “dcbabcd”.

分析：这道题我的思路是比较简单的，要补全回文，那就从中间开始，判断是否是有效构造回文的交界点。如果不是的话，就base–，这里可能要注意的一点是对于每一个base而言，都有两种可能，一是奇数型，二是偶数型（自己命名的，，）。这种做法时间复杂度为O(n2)，据说还有O(n)的解决方案，是使用的Manacher算法。

运行时间：336ms

public class Solution {      public String shortestPalindrome(String s) {        char[] chars = s.toCharArray();         if(chars.length<=0)            return "";        int length = chars.length;        int base = (length-1)/2;        boolean isOdd = true;        while (base>=0){            if(isValid(chars,base,base+1)) {                isOdd = false;                break;            }            if(isValid(chars,base,base))                break;            base--;        }        StringBuilder builder = new StringBuilder();        int index = length-1;        while(index>=base){            builder.append(chars[index--]);        }        int oneSize = builder.length();        if(isOdd)//如果是奇数            index = oneSize-2;        else{            builder.deleteCharAt(oneSize-1);            index = oneSize-2;        }        while(index>=0)            builder.append(builder.charAt(index--));        return builder.toString();    }    private boolean isValid(char[] chars,int left,int right){        while(left>=0){            if(right>=chars.length || chars[left]!=chars[right])                return false;            left--;right++;        }        return true;    }}

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