680. Valid Palindrome II

这道题的暴力解法很简单，先check如果不删除任何字符，是否字符串是回文，如果不是，再挨个删除每个字符，check删除字符之后是否是回文。

时间复杂度O(n2), n是字符串s的长度，字符串很长的情况下会TLE，算法如下：

class Solution {     public boolean validPalindrome(String s) {         if(checkPalindrom(s))             return true;         for(int i=0;i<s.length();i++){             if(checkPalindrom(s.substring(0, i)+s.substring(i+1)))                 return true;         }         return false;              }          private boolean checkPalindrom(String s){         int i=0, j=s.length()-1;         while(i<j){             if(s.charAt(i)==s.charAt(j)){                 i++;                 j--;             }             else                 return false;         }         return true;     } }

上面的算法，每删掉一个字符，就要从头到尾重新挨个字符check一遍字符串，其实这个过程可以one pass，而且这个算法可以延伸到去掉任意数量的字符.

时间复杂度O(n), 算法如下：

class Solution {     public boolean validPalindrome(String s) {         return helper(s, 0, s.length()-1, 0);     }          private boolean helper(String s, int i, int j, int sum){         if(sum>1)             return false;         while(i<j && s.charAt(i)==s.charAt(j)){             i++;             j--;         }         if(i>=j)             return true;         return helper(s, i+1, j, sum+1) || helper(s, i, j-1, sum+1);     } }

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