Min Stack

题目：

Design a stack that supports push, pop, top, and retrieving the minimum element in
constant time.
 push(x) – Push element x onto stack.
 pop() – Removes the element on top of the stack.
 top() – Get the top element.
 getMin() – Retrieve the minimum element in the stack

解答：

public class MinStack {
    private Stack<Integer> stack = new Stack<>();
    private Stack<Integer> minStack = new Stack<>();

    public void push(int x) {
        stack.push(x);
        if(minStack.isEmpty() || x <= minStack.peek()) {
            minStack.push(x);
        }
    }

    public void pop() {
        if(stack.pop().equals(minStack.peek())) {
            minStack.pop();
        }
    }

    public int top() {
        return stack.peek();
    }

    public int getMin() {
        return minStack.peek();
    }
}