[Leetcode] 680. Valid Palindrome II 解题报告

题目：

Given a non-empty string s, you may delete at most one character. Judge whether you can make it a palindrome.

Example 1:

Input: "aba"
Output: True

Example 2:

Input: "abca"
Output: True
Explanation: You could delete the character 'c'.

Note:

1. The string will only contain lowercase characters a-z. The maximum length of the string is 50000.

思路：

我们采用递归的思路：如果s[left] = s[right]，那么递归检查内部就可以；否则我们必须删除掉s[left]或者s[right]，如果删除其中一个之后形成了回文串就说明可以；否则说明无法通过删除掉其中一个来生成回文串。

代码：

class Solution {
public:
    bool validPalindrome(string s) {
        int length = s.length();
        if (length == 0) {
            return true;
        }
        else {
            return validPalindrome(s, 0, length - 1);
        }
    }
private:
    bool validPalindrome(string &s, int left, int right) {
        if (left >= right) {
            return true;
        }
        if (s[left] == s[right]) {      // we do not need to remove
            return validPalindrome(s, left + 1, right - 1);
        }
        else {                          // we have to remove s[left] or s[right]
            return isPalindrome(s, left + 1, right) || isPalindrome(s, left, right - 1);
        }
    }
    bool isPalindrome(string &s, int left, int right) {
        while (left < right) {
            if (s[left++] != s[right--]) {
                return false;
            }
        }
        return true;
    }
};