921. Minimum Add to Make Parentheses Valid
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, orIt can be written as AB (A concatenated with B), where A and B are valid strings, orIt 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:
S.length <= 1000
S only consists of ‘(‘ and ‘)’ characters.
题目地址
这是栈的经典应用
java 代码
class Solution {
public int minAddToMakeValid(String S) {
Stack<Character> stack = new Stack<>();
char[] c = S.toCharArray();
for(int i=0;i<c.length;i++){
if(c[i]==’)’ && !stack.empty() && stack.peek()=='(‘){
stack.pop();
}else{
stack.push(c[i]);
}
}
return stack.size();
}
}