116. 填充同一层的兄弟节点

struct TreeLinkNode {
  TreeLinkNode *left;
  TreeLinkNode *right;
  TreeLinkNode *next;
}

给定完美二叉树，

     1
   /  \
  2    3
 / \  / \
4  5  6  7
调用你的函数后，该完美二叉树变为：

     1 -> NULL
   /  \
  2 -> 3 -> NULL
 / \  / \
4->5->6->7 -> NULL

/**
 * 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) {
        TreeLinkNode level_start = root;
        while (level_start != null) {
            TreeLinkNode cur = level_start;
            while (cur != null) {
                if (cur.left != null) {
                    cur.left.next = cur.right;
                }
                if (cur.right != null && cur.next != null) {
                    cur.right.next = cur.next.left;
                }

                cur = cur.next;
            }
            level_start = level_start.left;
        }
    }
}