剑指 Offer 30. 包含min函数的栈

class MinStack {

    Stack<Integer> stack1;
    Stack<Integer> stack2;

    /** initialize your data structure here. */
    public MinStack() {
        stack1 = new Stack();
        stack2 = new Stack();
    }
    
    public void push(int x) {
        stack1.push(x);
        if(stack2.isEmpty()){
            stack2.push(x);
        }else if(x > stack2.peek()){
            stack2.push(stack2.peek());
        }else{
            stack2.push(x);
        }
    }
    
    public void pop() {
        stack1.pop();
        stack2.pop();
    }
    
    public int top() {
        return stack1.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();
 */