Sword finger offer 30 Stack containing min function

The finger of the sword Offer 30. contain min Function of the stack

The difficulty is simple 272

Defines the data structure of the stack , Please implement a in this type that can get the minimum elements of the stack min The function is in the stack , call min、push And pop The time complexity of O(1).

Example :

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.min();   -->  return  -3.
minStack.pop();
minStack.top();      -->  return  0.
minStack.min();   -->  return  -2.

class MinStack {

    /** initialize your data structure here. */
    Stack<Integer> stack1;
    Stack<Integer> stack2;
    public MinStack() {
        stack1=new Stack<>();
        stack2=new Stack<>();
    }
// push(x)  function ：  The focus is to keep the stack  B  The element is   Not strictly descending   Of .
    // take  x Push to stack  A （ namely  A.add(x) ）;
    // if  ①  Stack  B  It's empty   or  ② x  Less than or equal to   Stack  B  Top element of , Will  x  Push to stack  B （ namely  B.add(x) ）.
    public void push(int x) {
        stack1.add(x);
        if(stack2.isEmpty() || x<=stack2.peek()){
            stack2.add(x);
        }
    }
 //pop()  function ：  The focus is to keep the stack  A, B Of   Element consistency  .
    // Execution stack  A  Out of the stack （ namely  A.pop() ）, Record the out of stack element as  y ;
    // if  y  Equal stack  B  Top element of , Then execute stack  B  Out of the stack （ namely  B.pop() ）   
    public void pop() {
        //stack1.pop(); Redundant eject 
        if(stack1.pop().equals(stack2.peek())){
            stack2.pop();
        }
    }
    //top()  function ：  Directly back to the stack  A  The top element of the stack , Return  A.peek() .
    public int top() {
        return stack1.peek();
    }
 //min()  function ：  Directly back to the stack  B  The top element of the stack , Return  B.peek() .   
    public int min() {
        return stack2.peek();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.min();
 */