【leetcode】面试题 03.01. 三合一

三合一。描述如何只用一个数组来实现三个栈。

你应该实现push(stackNum, value)、pop(stackNum)、isEmpty(stackNum)、peek(stackNum)方法。stackNum表示栈下标，value表示压入的值。

构造函数会传入一个stackSize参数，代表每个栈的大小。

示例1:

输入：
[“TripleInOne”, “push”, “push”, “pop”, “pop”, “pop”, “isEmpty”]
[[1], [0, 1], [0, 2], [0], [0], [0], [0]]
输出：
[null, null, null, 1, -1, -1, true]
说明：当栈为空时pop, peek返回-1，当栈满时push不压入元素。
示例2:

输入：
[“TripleInOne”, “push”, “push”, “push”, “pop”, “pop”, “pop”, “peek”]
[[2], [0, 1], [0, 2], [0, 3], [0], [0], [0], [0]]
输出：
[null, null, null, null, 2, 1, -1, -1]

class TripleInOne {

    private int[] mystack1 = null;
    private int[] mystack2 = null;
    private int[] mystack3 = null;
    private int top1;
    private int top2;
    private int top3;
    public TripleInOne(int stackSize) {
        mystack1 = new int[stackSize];
        mystack2 = new int[stackSize];
        mystack3 = new int[stackSize];
    }
    
    public void push(int stackNum, int value) {
        // 判断是哪个栈需要压入元素，当栈中的元素已经和栈长度一样的时候则不压入栈
        if(stackNum == 0 && top1 < mystack1.length){
            mystack1[top1 ++] = value;
        }
        if(stackNum == 1 && top2 < mystack2.length){
            mystack2[top2 ++] = value;
        }
        if(stackNum == 2 && top3 < mystack3.length){
            mystack3[top3 ++] = value;
        }
    }
    
    public int pop(int stackNum) {
        if(stackNum == 0 && top1 > 0) {
            top1 = top1 - 1;
            return mystack1[top1];
        }
        if(stackNum == 1 && top2 > 0){
            top2 = top2 - 1;
            return mystack2[top2];
        }
        if(stackNum == 2 && top3 > 0){
           top3 = top3 - 1;
            return mystack3[top3];
        }
        return -1;
    }
    
    public int peek(int stackNum) {
        if(stackNum == 0 && top1 > 0) {
            return mystack1[top1 -1];
        }
        if(stackNum == 1 && top2 > 0){
            return mystack2[top2 -1];
        }
        if(stackNum == 2 && top3 > 0){
            return mystack3[top3 -1];
        }
        return -1;
    }
    
    public boolean isEmpty(int stackNum) {
        if(stackNum == 0) {
            return top1 == 0 ? true : false;
        }
        if(stackNum == 1){
            return top2 == 0 ? true : false;
        }
        return top3 == 0 ? true : false;
    }
}

/**
 * Your TripleInOne object will be instantiated and called as such:
 * TripleInOne obj = new TripleInOne(stackSize);
 * obj.push(stackNum,value);
 * int param_2 = obj.pop(stackNum);
 * int param_3 = obj.peek(stackNum);
 * boolean param_4 = obj.isEmpty(stackNum);
 */