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 withB
), whereA
andB
are valid strings, or - It can be written as
(A)
, whereA
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:
S.length <= 1000
S
only consists of'('
and')'
characters.
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这个题是栈的基础题,不过可以用O(n)的时间复杂度和O(1)的空间复杂度来解这个问题。参考题解:https://leetcode.com/articles/minimum-add-to-make-parentheses-valid/
C++代码:
class Solution { public: int minAddToMakeValid(string S) { int ans = 0,bal = 0; for(int i = 0; i < S.size(); i++){ bal += S[i] == '('?1:-1; if(bal == -1){ ans++; bal++; } } return ans+bal; } };