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Posts tagged as “queue”

花花酱 LeetCode 133. Clone Graph

Given the head of a graph, return a deep copy (clone) of the graph. Each node in the graph contains a label (int) and a list (List[UndirectedGraphNode]) of its neighbors. There is an edge between the given node and each of the nodes in its neighbors.
OJ’s undirected graph serialization (so you can understand error output):

Nodes are labeled uniquely.We use # as a separator for each node, and , as a separator for node label and each neighbor of the node.

As an example, consider the serialized graph {0,1,2#1,2#2,2}.

The graph has a total of three nodes, and therefore contains three parts as separated by #.

  1. First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
  2. Second node is labeled as 1. Connect node 1 to node 2.
  3. Third node is labeled as 2. Connect node 2 to node 2 (itself), thus forming a self-cycle.

Visually, the graph looks like the following:

      / \
     /   \
    0 --- 2
         / \

Note: The information about the tree serialization is only meant so that you can understand error output if you get a wrong answer. You don’t need to understand the serialization to solve the problem.

Solution: Queue + Hashtable

Time complexity: O(V+E)
Space complexity: O(V+E)


花花酱 LeetCode 933. Number of Recent Calls


Write a class RecentCounter to count recent requests.

It has only one method: ping(int t), where t represents some time in milliseconds.

Return the number of pings that have been made from 3000 milliseconds ago until now.

Any ping with time in [t - 3000, t] will count, including the current ping.

It is guaranteed that every call to ping uses a strictly larger value of t than before.

Example 1:

Input: inputs = ["RecentCounter","ping","ping","ping","ping"], inputs = [[],[1],[100],[3001],[3002]]
Output: [null,1,2,3,3]


  1. Each test case will have at most 10000 calls to ping.
  2. Each test case will call ping with strictly increasing values of t.
  3. Each call to ping will have 1 <= t <= 10^9.

Solution: Queue

Use a FIFO queue to track all the previous pings that are within 3000 ms to current.

Time complexity: Avg O(1), Total O(n)

Space complexity: O(n)


花花酱 LeetCode 622. Design Circular Queue

Design your implementation of the circular queue. The circular queue is a linear data structure in which the operations are performed based on FIFO (First In First Out) principle and the last position is connected back to the first position to make a circle. It is also called “Ring Buffer”.

One of the benefits of the circular queue is that we can make use of the spaces in front of the queue. In a normal queue, once the queue becomes full, we cannot insert the next element even if there is a space in front of the queue. But using the circular queue, we can use the space to store new values.

Your implementation should support following operations:

  • MyCircularQueue(k): Constructor, set the size of the queue to be k.
  • Front: Get the front item from the queue. If the queue is empty, return -1.
  • Rear: Get the last item from the queue. If the queue is empty, return -1.
  • enQueue(value): Insert an element into the circular queue. Return true if the operation is successful.
  • deQueue(): Delete an element from the circular queue. Return true if the operation is successful.
  • isEmpty(): Checks whether the circular queue is empty or not.
  • isFull(): Checks whether the circular queue is full or not.


MyCircularQueue circularQueue = new MyCircularQueue(3); // set the size to be 3
circularQueue.enQueue(1);  // return true
circularQueue.enQueue(2);  // return true
circularQueue.enQueue(3);  // return true
circularQueue.enQueue(4);  // return false, the queue is full
circularQueue.Rear();  // return 3
circularQueue.isFull();  // return true
circularQueue.deQueue();  // return true
circularQueue.enQueue(4);  // return true
circularQueue.Rear();  // return 4


  • All values will be in the range of [0, 1000].
  • The number of operations will be in the range of [1, 1000].
  • Please do not use the built-in Queue library.

Solution: Simulate with an array

We need a fixed length array, and the head location as well as the size of the current queue.

We can use q[head] to access the front, and q[(head + size – 1) % k] to access the rear.

Time complexity: O(1) for all the operations.
Space complexity: O(k)




花花酱 LeetCode 225. Implement Stack using Queues



Implement the following operations of a stack using queues.

  • push(x) — Push element x onto stack.
  • pop() — Removes the element on top of the stack.
  • top() — Get the top element.
  • empty() — Return whether the stack is empty.


  • You must use only standard operations of a queue — which means only push to backpeek/pop from frontsize, and is empty operations are valid.
  • Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
  • You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).


Using a single queue, for every push, shift the queue (n – 1) times such that the last element becomes the first element in the queue.


push(1): q: [1]

push(2): q: [1, 2] -> [2, 1]

push(3): q: [2, 1, 3] -> [1, 3, 2] -> [3, 2, 1]

push(4): q: [3, 2, 1, 4] -> [2, 1, 4, 3] -> [1, 4, 3, 2] -> [4, 3, 2, 1]


Time complexity:

Push: O(n)

Pop/top/empty: O(1)

Space complexity: O(n)



花花酱 LeetCode 239. Sliding Window Maximum



Given an array nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see the k numbers in the window. Each time the sliding window moves right by one position.

For example,
Given nums = [1,3,-1,-3,5,3,6,7], and k = 3.

Therefore, return the max sliding window as [3,3,5,5,6,7].

You may assume k is always valid, ie: 1 ≤ k ≤ input array’s size for non-empty array.

Follow up:
Could you solve it in linear time?




Solution 1: Brute Force

Time complexity: O((n – k + 1) * k)

Space complexity: O(1)




Solution 2: BST

Time complexity: O((n – k + 1) * logk)

Space complexity: O(k)


Solution 3: Monotonic Queue

Time complexity: O(n)

Space complexity: O(k)


C++ V2

C++ V3



Python3 V2