Minimum Remove to Make Valid Parentheses

class Solution {
public:
    string minRemoveToMakeValid(string s) {
        stack<int> opens;
        vector<bool> remove(s.size(), false);
        for (int i = 0; i < (int)s.size(); ++i) {
            if (s[i] == '(') opens.push(i);
            else if (s[i] == ')') {
                if (!opens.empty()) opens.pop();
                else remove[i] = true;
            }
        }
        while (!opens.empty()) { remove[opens.top()] = true; opens.pop(); }
        string ans;
        ans.reserve(s.size());
        for (int i = 0; i < (int)s.size(); ++i)
            if (!remove[i]) ans.push_back(s[i]);
        return ans;
    }
};

func minRemoveToMakeValid(s string) string {
    remove := make([]bool, len(s))
    var opens []int
    for i := 0; i < len(s); i++ {
        switch s[i] {
        case '(':
            opens = append(opens, i)
        case ')':
            if len(opens) > 0 {
                opens = opens[:len(opens)-1]
            } else {
                remove[i] = true
            }
        }
    }
    for _, idx := range opens {
        remove[idx] = true
    }
    b := make([]byte, 0, len(s))
    for i := 0; i < len(s); i++ {
        if !remove[i] {
            b = append(b, s[i])
        }
    }
    return string(b)
}

class Solution:
    def minRemoveToMakeValid(self, s: str) -> str:
        remove = [False] * len(s)
        opens: list[int] = []
        for i, ch in enumerate(s):
            if ch == '(':
                opens.append(i)
            elif ch == ')':
                if opens:
                    opens.pop()
                else:
                    remove[i] = True
        for idx in opens:
            remove[idx] = True
        return ''.join(ch for i, ch in enumerate(s) if not remove[i])

function minRemoveToMakeValid(s) {
    const remove = new Array(s.length).fill(false);
    const opens = [];
    for (let i = 0; i < s.length; i++) {
        const ch = s[i];
        if (ch === '(') opens.push(i);
        else if (ch === ')') {
            if (opens.length) opens.pop();
            else remove[i] = true;
        }
    }
    for (const idx of opens) remove[idx] = true;
    let out = '';
    for (let i = 0; i < s.length; i++) {
        if (!remove[i]) out += s[i];
    }
    return out;
}

class Solution {
    public String minRemoveToMakeValid(String s) {
        int n = s.length();
        boolean[] remove = new boolean[n];
        Deque<Integer> opens = new ArrayDeque<>();
        for (int i = 0; i < n; i++) {
            char ch = s.charAt(i);
            if (ch == '(') opens.push(i);
            else if (ch == ')') {
                if (!opens.isEmpty()) opens.pop();
                else remove[i] = true;
            }
        }
        while (!opens.isEmpty()) remove[opens.pop()] = true;
        StringBuilder sb = new StringBuilder(n);
        for (int i = 0; i < n; i++) {
            if (!remove[i]) sb.append(s.charAt(i));
        }
        return sb.toString();
    }
}

function minRemoveToMakeValid(s: string): string {
    const remove: boolean[] = new Array(s.length).fill(false);
    const opens: number[] = [];
    for (let i = 0; i < s.length; i++) {
        const ch = s[i];
        if (ch === '(') opens.push(i);
        else if (ch === ')') {
            if (opens.length) opens.pop();
            else remove[i] = true;
        }
    }
    for (const idx of opens) remove[idx] = true;
    let out = '';
    for (let i = 0; i < s.length; i++) {
        if (!remove[i]) out += s[i];
    }
    return out;
}