[swift] LeetCode 394. Decode String

Given an encoded string, return it’s decoded string.

The encoding rule is: k[encoded_string] , where the  encoded_string inside the square brackets is being repeated exactly  k times. Note that  k is guaranteed to be a positive integer.

You may assume that the input string is always valid; No extra white spaces, square brackets are well-formed, etc.

Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k . For example, there won’t be input like  3a or  2[4] .

Examples:

s = "3[a]2[bc]", return "aaabcbc".
s = "3[a2[c]]", return "accaccacc".
s = "2[abc]3[cd]ef", return "abcabccdcdcdef".

func decodeString(_ s: String) -> String {
    var stack = [String]()
    s.forEach {
        switch $0 {
        case "]":
            var s = "", i = ""
            while let c = stack.popLast(), c != "[" {
                s = "\(c)\(s)"
            }
            while let sl = stack.last, nil != Int("\(sl)"), let c = stack.popLast() {
                i = "\(c)\(i)"
            }
            stack.append(String(repeating: s, count: Int(i)!))
        default:
            stack.append(String($0))
        }
    }
 
    return stack.joined()
}

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