921. Minimum Add to Make Parentheses Valid

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Given a string S of '(' and ')' parentheses, we add the minimum number of parentheses ( '(' or ')', and in any positions ) so that the resulting parentheses string is valid.

Formally, a parentheses string is valid if and only if:

  • It is the empty string, or
  • It can be written as AB (A concatenated with B), where A and B are valid strings, or
  • It can be written as (A), where A is a valid string.

Given a parentheses string, return the minimum number of parentheses we must add to make the resulting string valid.

Example 1:

Input: "())"
Output: 1

Example 2:

Input: "((("
Output: 3

Example 3:

Input: "()"
Output: 0

Example 4:

Input: "()))(("
Output: 4

Note:

  1. S.length <= 1000
  2. S only consists of '(' and ')' characters.

思路:用stack模拟一遍,stack里面只放(,遇到),如果stack为空,就+1,否则pop stack

class Solution(object):
    def minAddToMakeValid(self, S):
        """
        :type S: str
        :rtype: int
        """
        res=0
        st=[]
        for s in S:
            if s=='(': 
                st.append(s)
            else:
                if not st: res+=1
                else: st.pop()
        res+=len(st)
        return res
    
s=Solution()
print(s.minAddToMakeValid("())"))
print(s.minAddToMakeValid("((("))
print(s.minAddToMakeValid("()"))
print(s.minAddToMakeValid("()))(("))
        

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转载自blog.csdn.net/zjucor/article/details/83045419