题目

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]

提示：

• 0 <= stackNum <= 2

题解

class TripleInOne {
    /**
     * 数组存储栈
     * 0 3 6..表示第一个栈
     * 1 4 7..表示第二个栈
     * 2 5 8..表示第三个栈
     */
    private int[] data;
    private int top1 = -3;
    private int top2 = -2;
    private int top3 = -1;

    public TripleInOne(int stackSize) {
        this.data = new int[stackSize * 3];
    }

    public void push(int stackNum, int value) {
        int next = this.getTop(stackNum) + 3;
        if (next < this.data.length) {
            data[next] = value;
            this.addTop(stackNum);
        }
    }

    public int pop(int stackNum) {
        int t = this.getTop(stackNum);
        if (t < 0) {
            return -1;
        }

        this.minusTop(stackNum);

        return data[t];
    }

    public int peek(int stackNum) {
        int t = this.getTop(stackNum);
        return t < 0 ? -1 : this.data[t];
    }

    public boolean isEmpty(int stackNum) {
        return this.getTop(stackNum) < 0;
    }

    private int getTop(int stackNum) {
        switch (stackNum) {
            case 0:
                return this.top1;
            case 1:
                return this.top2;
            default:
                return this.top3;
        }
    }

    private void addTop(int stackNum) {
        switch (stackNum) {
            case 0:
                this.top1 += 3;
                break;
            case 1:
                this.top2 += 3;
                break;
            default:
                this.top3 += 3;
                break;
        }
    }

    private void minusTop(int stackNum) {
        switch (stackNum) {
            case 0:
                this.top1 -= 3;
                break;
            case 1:
                this.top2 -= 3;
                break;
            default:
                this.top3 -= 3;
                break;
        }
    }
}