Leetcode 921. Minimum Add to Make Parentheses Valid (stack 题)

Leetcode 921. Minimum Add to Make Parentheses Valid
Medium
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 ‘)’.

解法1：用stack.

class Solution {
public:
    int minAddToMakeValid(string s) {
        stack<char> stk;
        int leftNeeded = 0, rightNeeded = 0;
        for (auto c : s) {
            if (c == '(') {
                stk.push(c);
            } else {
                if (stk.empty()) leftNeeded++;
                else stk.pop();
            }
        }
        return leftNeeded + stk.size();
    }
};

解法2：不用栈也可以。

class Solution {
public:
    int minAddToMakeValid(string s) {        
        int leftNeeded = 0, rightNeeded = 0;
        for (auto c : s) {
            if (c == '(') {
                rightNeeded++; 
            } else {
                rightNeeded--;
                if (rightNeeded == -1) {
                    leftNeeded++;
                    rightNeeded = 0;
                }
            }
        }
        return leftNeeded + rightNeeded;
    }
};