117. Populating Next Right Point

Follow up for problem "Populating Next Right Pointers in Each Node".

What if the given tree could be any binary tree? Would your previous solution still work?

Note:

You may only use constant extra space.
For example,
Given the following binary tree,

     1
   /  \
  2    3
 / \    \
4   5    7

After calling your function, the tree should look like:

     1 -> NULL
   /  \
  2 -> 3 -> NULL
 / \    \
4-> 5 -> 7 -> NULL

这道题目可以将两个队列结合起来进行层次遍历。

import java.util.*;
/**
 * Definition for binary tree with next pointer.
 * public class TreeLinkNode {
 *     int val;
 *     TreeLinkNode left, right, next;
 *     TreeLinkNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void connect(TreeLinkNode root) {
        if(root == null) {
            return;
        }
        Queue<TreeLinkNode> q1 = new LinkedList<>();
        Queue<TreeLinkNode> q2 = new LinkedList<>();
        q1.offer(root);
        while(!q1.isEmpty()) {
            TreeLinkNode temp = q1.poll();
            if(q1.isEmpty()) {
                temp.next = null;
            } else {
                temp.next = q1.peek();
            }
            if(temp.left != null) {
                q2.offer(temp.left);
            }
            if(temp.right != null) {
                q2.offer(temp.right);
            }
            if(q1.isEmpty() && !q2.isEmpty()) {
                while(!q2.isEmpty()) {
                    q1.offer(q2.poll());
                }
            }
        }
    }
}