题目：牛客网

解题思路：

利用层序遍历

 

import java.util.LinkedList;
import java.util.Queue;
/**
 * 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> queue = new LinkedList<TreeLinkNode>();
        queue.add(root);
        while(!queue.isEmpty()){
            int len = queue.size();
            for(int i = 0; i < len; i++){
                TreeLinkNode head = queue.poll();
                if(i == len-1)
                    head.next = null;
                else
                    head.next = queue.peek();
                if(head.left != null)
	    			queue.add(head.left);
                if(head.right != null)
                    queue.add(head.right);
            }
        }
    }
}