_155_MinStack.java

package leetcode_1To300;

import java.util.Stack;

/**
 * 本代码来自 Cspiration，由 @Cspiration 提供
 * 题目来源：http://leetcode.com
 * - Cspiration 致力于在 CS 领域内帮助中国人找到工作，让更多海外国人受益
 * - 现有课程：Leetcode Java 版本视频讲解（1-900题）（上）（中）（下）三部
 * - 算法基础知识（上）（下）两部；题型技巧讲解（上）（下）两部
 * - 节省刷题时间，效率提高2-3倍，初学者轻松一天10题，入门者轻松一天20题
 * - 讲师：Edward Shi
 * - 官方网站：https://cspiration.com
 * - 版权所有，转发请注明出处
 */
public class _155_MinStack {

    /** initialize your data structure here. */
    /**
     * 155. Min StackTest
     * 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.

     Example:
     _155_MinStack minStack = new _155_MinStack();
     minStack.push(-2);
     minStack.push(0);
     minStack.push(-3);
     minStack.getMin();   --> Returns -3.
     minStack.pop();
     minStack.top();      --> Returns 0.
     minStack.getMin();   --> Returns -2.

     在-128~127的Integer值并且以Integer x = value;的方式赋值的Integer值在进行==和equals比较时，都会返回true，
     因为Java里面对处在在-128~127之间的Integer值，用的是原生数据类型int，会在内存里供重用，
     也就是说这之间的Integer值进行==比较时只是进行int原生数据类型的数值比较，而超出-128~127的范围，进行==比较时是进行地址及数值比较

     time : O(1)
     space : O(n)
     */

    private Stack<Integer> stack;
    private Stack<Integer> minStack;


    public _155_MinStack() {
        stack = new Stack<>();
        minStack = new Stack<>();
    }

    public void push(int x) {
        stack.push(x);
        if (!minStack.isEmpty()) {
            int min = minStack.peek();
            if (x <= min) {
                minStack.push(x);
            }
        } else {
            minStack.push(x);
        }
    }

    public void pop() {
        int x = stack.pop();
        if (x == minStack.peek()) {
            minStack.pop();
        }
    }

    public int top() {
        return stack.peek();
    }

    public int getMin() {
        return minStack.peek();
    }
}

class MinStack2 {

    /**
     minStack.push(-2);
     minStack.push(0);
     minStack.push(-3);
     minStack.getMin();   --> Returns -3.
     minStack.pop();
     minStack.top();      --> Returns 0.
     minStack.getMin();   --> Returns -2.

     */

    Stack<Integer> stack;
    int min;

    public MinStack2() {
        stack = new Stack<>();
        min = Integer.MAX_VALUE;
    }

    public void push(int x) {
        if (x <= min) {
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }

    public void pop() {
        if (stack.pop() == min) {
            min = stack.pop();
        }
    }

    public int top() {
        return stack.peek();
    }

    public int getMin() {
        return min;
    }
}