[swift] LeetCode 20. Valid Parentheses

Total Accepted: 103858 Total Submissions: 355021 Difficulty: Easy

Given a string containing just the characters ‘(‘, ‘)’, ‘{‘, ‘}’, ‘[‘ and ‘]’, determine if the input string is valid.

The brackets must close in the correct order, “()” and “()[]{}” are all valid but “(]” and “([)]” are not.

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func isValid(_ s: String) -> Bool {
    var stack = [Character](), b = true
 
    func compare(_ c: Character) -> Bool {
        guard let t = stack.popLast()  else { return false }
        return (c == ")" && t == "(") || (c == "]" && t == "[" ) || (c == "}" && t == "{")
    }
 
    loop: for c in s {
        switch c {
        case "(", "[", "{":
            stack.append(c)
        default:
            b = compare(c)
            if !b { break loop }
        }
    }
    
    return b && stack.isEmpty
}

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