## Populating Next Right Pointers in Each Node

### 描述

Given a binary tree

struct TreeLinkNode {
int val;
TreeLinkNode *left, *right, *next;
TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {}
};


Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.

Initially, all next pointers are set to NULL.

Note:

• You may only use constant extra space.
• You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).

For example,

         1
/  \
2    3
/ \  / \
4  5  6  7


After calling your function, the tree should look like:

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


### 代码

// Populating Next Right Pointers in Each Node
// 时间复杂度O(n)，空间复杂度O(logn)
public class Solution {
public void connect(TreeLinkNode root) {
connect(root, null);
}
private static void connect(TreeLinkNode root, TreeLinkNode sibling) {
if (root == null) return;
else root.next = sibling;

connect(root.left, root.right);
if (sibling != null) connect(root.right, sibling.left);
else connect(root.right, null);
}
}