/**
 * @param {string} s
 * @return {boolean}
 */
var isValid = function(s) {
    let stack = [];
    for (let i = 0; i < s.length; i++) {
        let char = s[i];
        if (char === '(' || char === '{' || char === '[') {
            stack.push(char)
        } else {
            if (!stack.length) return false;
            let prevChar = stack.pop();
            if (char === ')' && prevChar !== '(') {
                return false;
            }
            if (char === '}' && prevChar !== '{') {
                return false;
            }
            if (char === ']' && prevChar !== '[') {
                return false;
            }
        }
    }
    
    return !stack.length;
};