剑指Offer30. 包含min函数的栈

方法一：使用辅助栈

题目要求各种操作的时间复杂度都为O(1)，所以要想到用空间换时间的方向，用一个辅助栈来记录数据栈每次更新时的最小值的变化情况。

/**
 * initialize your data structure here.
 */
var MinStack = function() {
    this.dataStack = [];
    this.minDataStack = [Infinity];//Infinity的作用是当第一次比较时即有值可比
};

/** 
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function(x) {
    this.dataStack.push(x);
    this.minDataStack.push(Math.min(x, this.minDataStack[this.minDataStack.length - 1]));
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function() {
    this.dataStack.pop();
    this.minDataStack.pop();
};

/**
 * @return {number}
 */
MinStack.prototype.top = function() {
    return this.dataStack[this.dataStack.length - 1];
};

/**
 * @return {number}
 */
MinStack.prototype.min = function() {
    //console.log(this.dataStack.length);
    //console.log(this.minDataStack);
    return this.minDataStack[this.dataStack.length]; //return this.minDataStack[this.minDataStack.length - 1];
};

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

Infinity的用法

暂时不太清楚。  //2021.9.1

 

 方法二：记录差值

数据栈中记录当前元素与最小值的差值，通过栈中数据的正负来判断数据是否为最小值，栈中第一个数据需要特殊

/**
 * initialize your data structure here.
 */
var MinStack = function() {
    this.dataStack = [];
    this.min_val = 0;
};

/** 
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function(x) {
    //若栈为空
    if(this.dataStack.length == 0) {
        this.dataStack.push(x);
        this.min_val = x;
    }else {//栈不为空
        //记录x与min_val的差值
        this.dataStack.push(x - this.min_val);
        //更新最小值
        this.min_val = Math.min(x, this.min_val);
        //console.log(this.min_val);
    }
    
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function() {
    //若当前栈中数据小于0，则表示出栈后最小值需要更新
    //这里不需要再判断栈长度是否为1，因为push时若为空栈会自动更新栈和最小值
    if(this.dataStack[this.dataStack.length - 1] < 0) {
        this.min_val -= this.dataStack[this.dataStack.length - 1];
    }
    this.dataStack.pop();
};

/**
 * @return {number}
 */
MinStack.prototype.top = function() {
    if(this.dataStack.length == 1) {
        //若此时栈长度为1
        return this.dataStack[this.dataStack.length - 1];
    }else if (this.dataStack[this.dataStack.length - 1] > 0) {
        //若此时栈顶元素为正，即表示栈顶元素不是最小值
        return this.dataStack[this.dataStack.length - 1] + this.min_val;
    }else {
        return this.min_val;
    }
};

/**
 * @return {number}
 */
MinStack.prototype.min = function() {
    return this.min_val;
};

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