https://leetcode-cn.com/problems/bao-han-minhan-shu-de-zhan-lcof/submissions/

class MinStack {
    var main = Array<Int>()
    //計入push過的最小值
    var sub = Array<Int>()
    
    init() {
        
    }
    
    func push(_ x: Int) {
        main.append(x)
        if sub.isEmpty || sub.last! >= x{
            sub.append(x)
        }
    }
    
    func pop() {
        if main.last! == sub.last! {
            sub.removeLast()
        }
        main.removeLast()
    }
    
    func top() -> Int {
        return main.last!
    }
    
    func min() -> Int {
        return sub.last!
    }
}