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//implement a queue using stack public class Solution { Stack stack1 = new Stack(); Stack stack2 = new Stack(); //pop: dequeue in stack1 public void dequeue() { if (stack1.peek()

Use two queues to implement stack. q2 is used to make sure the new element is always at the front of the q1. public class Solution { Queue q1 = new LinkedList(); Queue q2 = new LinkedList(

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next leve

Populating Next Right Pointers in Each Node Given a binary tree struct TreeLinkNode { TreeLinkNode *left; TreeLinkNode *right; TreeLinkNode *next; } P