[leetcode]678. Valid Parenthesis String验证有效括号字符串

Given a string containing only three types of characters: '(', ')' and '*', write a function to check whether this string is valid. We define the validity of a string by these rules:

1. Any left parenthesis '(' must have a corresponding right parenthesis ')'.
2. Any right parenthesis ')' must have a corresponding left parenthesis '('.
3. Left parenthesis '(' must go before the corresponding right parenthesis ')'.
4. '*' could be treated as a single right parenthesis ')' or a single left parenthesis '(' or an empty string.
5. An empty string is also valid.

Example 1:

Input: "()"
Output: True

Example 2:

Input: "(*)"
Output: True

Example 3:

Input: "(*))"
Output: True

题目

验证有效括号字符串

思路

recursion

类似Leetcode 22 Generate Parenthesis 思路

代码

 class Solution {
     public boolean checkValidString(String s) {
         return check(s, 0, 0);
     }

     private boolean check(String s, int start, int count) {
         if (count < 0) return false;

         for (int i = start; i < s.length(); i++) {
             char c = s.charAt(i);
             if (c == '(') {
                 count++;
             }
             else if (c == ')') {
                 if (count <= 0) return false;
                 count--;
             }
             else if (c == '*') {
                 //1. *  for '(' --> (*))
                 //2. *  for ')' --> ((*)
                 //3. *  for empty string -->  (*)
                 return check(s, i + 1, count + 1) || check(s, i + 1, count - 1) || check(s, i + 1, count);
             }
         }

         return count == 0;
     }

 }