4.栈

栈的数据结构特点

#特点：后进先出
1.push  入栈
2.pop   将栈顶的数据出栈
3.peek  获取栈顶的数据
4.isEmpty  判断栈是否为空
5.size  返回栈的长度

/**
 * 入栈      push
 * 出栈      pop
 * peek     获取栈顶
 * isEmpty  判断栈是否为空
 * size     可以看栈里有多少个
*/
function Stack() {
    this.items = [];
}
Stack.prototype.push = function (val) {
    this.items.push(val);
}
Stack.prototype.pop = function () {
    var res = this.items.pop();
    return res;
}
Stack.prototype.peek = function () {
    return this.items[this.items.length - 1];
}
Stack.prototype.isEmpty = function () {
    return this.items.length == 0;
}
Stack.prototype.size = function () {
    return this.items.length;
}
var arr = new Stack();
arr.push(2)
arr.push(4)
console.log(arr);
arr.pop()
console.log(arr);
console.log(arr.peek());    // 2
console.log(arr.isEmpty()); //false
console.log(arr.size());    //1