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, 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

class Solution {
    public int minAddToMakeValid(String s) {
        Stack<Character> stack = new Stack<>();
        char[] chs = s.toCharArray();
        for (char ch: chs) {
            if (ch == '(') {
                stack.push(ch);
            } else {
                if (!stack.isEmpty() && stack.peek() == '(') {
                    stack.pop();
                } else {
                    stack.push(')');
                }
            }
        }
        return stack.size();
    }
    public int minAddToMakeValid(String S) {
        int left = 0, right = 0;
        for (int i = 0; i < S.length(); ++i) {
            if (S.charAt(i) == '(') {
                left++;
            } else if (left > 0) {
                left--;
            } else {
                right++;
            }
        }
        return left + right;
    }
}

这题也可以问 最少需要删除的bracket， 答案是一样的。