MinStack.swift

class MinStack {
    private var arr: [(Int, Int)]
    /** initialize your data structure here. */
    init() {
        arr = [(Int,Int)]()
    }
    
    func push(_ x: Int) {
        let prevMax = peek().1
        arr.append((x, min(prevMax,x)))
    }
    
    func pop() {
        guard arr.count > 0 else {
            return
        }
        arr.removeLast()
    }
    
    func top() -> Int {
        return peek().0
    }
    
    func getMin() -> Int {
        return peek().1
    }
    
    private func peek() -> (Int,Int){
        guard arr.count > 0 else {
            return (-1, Int.max)
        }
        return arr[arr.count - 1]
    }
}