Leetcode 921. Minimum Add to Make Parentheses Valid

栈的应用.

class Solution(object):def minAddToMakeValid(self, S):""":type S: str:rtype: int"""stack=[]for s in S:if s=='(':stack.append(s)else:if stack:if stack[-1]=='(':stack.pop()else:stack.append(s)else:stack.append(s)return len(stack)

这种方法是O(n)时间复杂度,O(n)空间复杂度,其实没有必要实现一个栈,因为右括号如果进入了栈就不可能再被消去了.所以就两个变量记录两边括号数就行了,空间复杂度为O(1):

class Solution(object):def minAddToMakeValid(self, S):""":type S: str:rtype: int"""left,right=0,0for s in S:if s=='(':left+=1else:if left>0:left-=1else:right+=1return left+right

转载于:https://www.cnblogs.com/zywscq/p/10519261.html

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