面试题 03.01. 三合一
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;
}
}
}