答案

/**
 * 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 ;
        }
        TreeLinkNode firstUpper=root;
        TreeLinkNode pUpper;
        TreeLinkNode firstBelow=null;
        TreeLinkNode pBelow=null;
        while(firstUpper!=null)
        {
            pUpper=firstUpper;
            while(pUpper!=null)
            {
                if(pUpper.left!=null)
                {
                    if(firstBelow==null)
                    {
                        firstBelow=pUpper.left;
                        pBelow=firstBelow;
                    }
                    else
                    {
                        pBelow.next=pUpper.left;
                        pBelow=pBelow.next;
                    }
                }
                if(pUpper.right!=null)
                {
                    if(firstBelow==null)
                    {
                        firstBelow=pUpper.right;
                        pBelow=firstBelow;
                    }
                    else
                    {
                        pBelow.next=pUpper.right;
                        pBelow=pBelow.next;
                    }
                }
                pUpper=pUpper.next;
            }
            firstUpper=firstBelow;
            firstBelow=null;
        }
    }
}