ValidParentheses.swift

/**
 * Question Link: https://leetcode.com/problems/valid-parentheses/
 * Primary idea: Use a stack to see whether the peek left brace is correspond to the current right one
 * Time Complexity: O(n), Space Complexity: O(n)
 */

class ValidParentheses {
    func isValid(_ s: String) -> Bool {
        var stack = [Character]()
        
        for char in s {
            if char == "(" || char == "[" || char == "{" {
                stack.append(char)
            } else if char == ")" {
                guard stack.count != 0 && stack.removeLast() == "(" else {
                    return false
                }
            } else if char == "]" {
                guard stack.count != 0 && stack.removeLast() == "[" else {
                    return false
                }
            } else if char == "}" {
                guard stack.count != 0 && stack.removeLast() == "{" else {
                    return false
                }
            }
        }
        
        return stack.isEmpty
    }
}