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：

1 public class MinStack { 2     private Stack
     
       stack = new Stack<>(); 3     private Stack
      
        minStack = new Stack<>(); 4  5     public void push(int x) { 6         stack.push(x); 7         if(minStack.isEmpty() || x <= minStack.peek()) { 8             minStack.push(x); 9         }10     }11 12     public void pop() {13         if(stack.pop().equals(minStack.peek())) {14             minStack.pop();15         }16     }17 18     public int top() {19         return stack.peek();20     }21 22     public int getMin() {23         return minStack.peek();24     }25 }