原文链接: https://leetcode-cn.com/problems/stack-of-plates-lcci

英文原文

Imagine a (literal) stack of plates. If the stack gets too high, it might topple. Therefore, in real life, we would likely start a new stack when the previous stack exceeds some threshold. Implement a data structure SetOfStacks that mimics this. SetOfStacks should be composed of several stacks and should create a new stack once the previous one exceeds capacity. SetOfStacks.push() and SetOfStacks.pop() should behave identically to a single stack (that is, pop() should return the same values as it would if there were just a single stack). Follow Up: Implement a function popAt(int index) which performs a pop operation on a specific sub-stack.

You should delete the sub-stack when it becomes empty. pop, popAt should return -1 when there's no element to pop.

Example1:

 Input: 
["StackOfPlates", "push", "push", "popAt", "pop", "pop"]
[[1], [1], [2], [1], [], []]
 Output: 
[null, null, null, 2, 1, -1]
 Explanation: 

Example2:

 Input: 
["StackOfPlates", "push", "push", "push", "popAt", "popAt", "popAt"]
[[2], [1], [2], [3], [0], [0], [0]]
 Output: 
[null, null, null, null, 2, 1, 3]

中文题目

堆盘子。设想有一堆盘子，堆太高可能会倒下来。因此，在现实生活中，盘子堆到一定高度时，我们就会另外堆一堆盘子。请实现数据结构SetOfStacks，模拟这种行为。SetOfStacks应该由多个栈组成，并且在前一个栈填满时新建一个栈。此外，SetOfStacks.push()和SetOfStacks.pop()应该与普通栈的操作方法相同（也就是说，pop()返回的值，应该跟只有一个栈时的情况一样）。 进阶：实现一个popAt(int index)方法，根据指定的子栈，执行pop操作。

当某个栈为空时，应当删除该栈。当栈中没有元素或不存在该栈时，pop，popAt 应返回 -1.

示例1:

 输入：
["StackOfPlates", "push", "push", "popAt", "pop", "pop"]
[[1], [1], [2], [1], [], []]
 输出：
[null, null, null, 2, 1, -1]

示例2:

 输入：
["StackOfPlates", "push", "push", "push", "popAt", "popAt", "popAt"]
[[2], [1], [2], [3], [0], [0], [0]]
 输出：
[null, null, null, null, 2, 1, 3]

通过代码

高赞题解

思路

1. 新建一个List<Stack>用来存放各个栈，而且栈的个数是动态变化的。
2. push的时候，可能需要新建一个栈，或者直接插入到最后一个栈中。
3. pop直接调用popAt方法。其中popAt方法需要处理的是弹出指定位置栈的栈顶元素。我们可以通过list拿到指定index的栈，拿到之后执行stack的pop操作即可。同时如果弹出栈顶元素之后，当前stack变成空了，需要将当前stack从list中移除。

class StackOfPlates {

        private List<Stack<Integer>> stackList;
        private int cap;

        public StackOfPlates(int cap) {
            stackList = new ArrayList<>();
            this.cap = cap;
        }

        public void push(int val) {
            if (cap <= 0) {
                return;
            }

            if (stackList.isEmpty() || stackList.get(stackList.size() - 1).size() == cap) {
                Stack<Integer> stack = new Stack<>();
                stack.push(val);
                stackList.add(stack);
                return;
            }

            stackList.get(stackList.size() - 1).push(val);
        }

        public int pop() {
            return popAt(stackList.size() - 1);
        }

        public int popAt(int index) {
            if (index < 0 || index >= stackList.size()) {
                return -1;
            }

            Stack<Integer> stack = stackList.get(index);
            if (stack.isEmpty()) {
                return -1;
            }

            int res = stack.pop();

            if (stack.isEmpty()) {
                stackList.remove(index);
            }

            return res;
        }
    }

统计信息

通过次数	提交次数	AC比率
9045	23349	38.7%

提交历史

提交时间	提交结果	执行时间	内存消耗	语言