921. Minimum Add to Make Parentheses Valid

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LeetCode

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Problem

A parentheses string is valid if and only if:

• It is the empty string,
• 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.

You are given a parentheses string s. In one move, you can insert a parenthesis at any position of the string.

• For example, if s = "()))", you can insert an opening parenthesis to be "(**(**)))" or a closing parenthesis to be "())**)**)".

Return the minimum number of moves required to make s valid.

Example 1:

Input: s = "())"
Output: 1

Example 2:

Input: s = "((("
Output: 3

Constraints:

• 1 <= s.length <= 1000
• s[i] is either '(' or ')'.

Code

class Solution {
    public int minAddToMakeValid(String S) {
        int left = 0;
        int res = 0;

        if (S == null || S.length() == 0)
            return 0;

        for (int i = 0; i < S.length(); i++) {
            char c = S.charAt(i);
            if (c == '(') {
                left++;
            } else {
                if (left > 0) {
                    left--;
                } else {
                    res++;
                }
            }
        }

        return res + left;
    }
}