[leedcode 155] Min Stack

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.

  class MinStack {
      //使用两个栈，一个正常使用，一个保存已经压栈中最小的数，每次push和pop注意两个栈同时操作
      private Stack<Integer> min=new Stack<Integer>();
      private Stack<Integer> stack=new Stack<Integer>();
      public void push(int x) {
          stack.push(x);
          if(min.empty()||x<min.peek()){//注意min判空
              min.push(x);
          }else{
              min.push(min.peek());
          }
      }
  
      public void pop() {
          stack.pop();
          min.pop();
          
      }
  
      public int top() {
          return stack.peek();
      }
  
      public int getMin() {
          return min.peek();
          
      }
  }