Valid Parentheses

LeetCode Problem #20

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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Constraints

• 1 <= s.length <= 104
• s consists of parentheses only '()[]{}'.

Solution

func isValid(_ s: String) -> Bool {
    var stack = [Character]()
    let pairs: [Character : Character] = ["(" : ")", "[" : "]", "{" : "}"]
    
    for char in s {
        switch char {
        case "(", "[", "{":
            stack.append(char)
        default:
            if !stack.isEmpty, char == pairs[stack.last!]! {
                stack.removeLast()
            } else {
                return false
            }
        }
    }
    return stack.isEmpty
}