Leetcode 921 Minimum Add to Make Parentheses Valid
思想:建立空栈,遍历字符串,遇到左括号压入栈中,遇到有括号进行判断,如果栈空直接压入栈中,如果与栈顶元素匹配则栈顶元素出栈,否则压栈,返回栈中元素的个数即为所求
class Solution(object):
def minAddToMakeValid(self, S):
"""
:type S: str
:rtype: int
"""
stack=[]
for x in S:
if x=='(':
stack.append(x)
elif x==')':
if not stack:
stack.append(x)
elif stack[-1]=='(':
stack.pop()
else:
stack.append(x)
return len(stack)