116. Populating Next Right Pointers in Each Node

Given a binary tree

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

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,

Given the following perfect binary tree,

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

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BFS

/**
* Definition for binary tree with next pointer.
* public class TreeLinkNode {
*     int val;
*     TreeLinkNode left, right, next;
*     TreeLinkNode(int x) { val = x; }
* }
*/
public class Solution {
private void sideConnect(TreeLinkNode root,TreeLinkNode side){
if(root.left==null)return;
if(side==null){
root.left.next=root.right;
root.right.next=null;
sideConnect(root.left,root.right);
sideConnect(root.right,null);
}else{
root.left.next=root.right;
root.right.next=side.left;
sideConnect(root.left,root.right);
sideConnect(root.right,side.left);
}
}
public void connect(TreeLinkNode root) {
if(root==null)return;
root.next=null;
if(root.left==null)return;
root.left.next=root.right;
root.right.next=null;
sideConnect(root.left,root.right);
sideConnect(root.right,null);
}
}