leetcode-155 最小栈 Java

https://leetcode-cn.com/problems/min-stack/solution/shi-yong-fu-zhu-zhan-tong-bu-he-bu-tong-bu-python-/

方法一：辅助栈和数据栈同步

import java.util.Stack;
class MinStack {
    private Stack<Integer> data;
    private Stack<Integer> helper;
    /** initialize your data structure here. */
    public MinStack() {
        data = new Stack<>();
        helper = new Stack<>();
    }
    
    public void push(int x) {
        data.add(x);
        if (helper.isEmpty() || helper.peek() >= x) {
            helper.add(x);
        } else {
            helper.add(helper.peek());
        }
    }
    
    public void pop() {
            helper.pop();
            data.pop();
    }
    
    public int top() {
            return data.peek();
    }
    
    public int getMin() {
        return helper.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.getMin();
 */

方法二：辅助栈和数据栈不同步

import java.util.Stack;
class MinStack {
    private Stack<Integer> data;
    private Stack<Integer> helper;
    /** initialize your data structure here. */
    public MinStack() {
        data = new Stack<>();
        helper = new Stack<>();
    }
    
    public void push(int x) {
        data.add(x);
        if (helper.isEmpty() || helper.peek() >= x) {
            helper.add(x);
        }
    }
    
    public void pop() {
        // int top = data.pop();
        // if (top == helper.peek()) {
        //     helper.pop();
        // }
        if ((int) helper.peek() == data.peek()) {
            helper.pop();
            data.pop();
        } else {
            data.pop();
        }
    }
    
    public int top() {
        return data.peek();
    }
    
    public int getMin() {
        return helper.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.getMin();
 */