117. Populating Next Right Pointers in Each Node II

Follow up for problem "Populating Next Right Pointers in Each Node".

What if the given tree could be any binary tree? Would your previous solution still work?

Note:

• You may only use constant extra space.

For example,
Given the following binary tree,

         1
       /  \
      2    3
     / \    \
    4   5    7

After calling your function, the tree should look like:

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

Analysis:

use 2 pointer to record head of each level and last linked element on the next level.

Complexity:

Time: O(N)

Space: O(1)

Code:

/**
 * 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 prev = null;
        TreeLinkNode head = null;
        TreeLinkNode cur = root;
        while(cur!=null)
        {
            while(cur!=null)
            {
                if(cur.left!=null)
                {
                    if(prev!=null)
                    {
                        prev.next = cur.left;
                    }
                    else if(head == null)
                    {
                        head = cur.left;
                    }
                    prev = cur.left;
                }

                if(cur.right!=null)
                {
                    if(prev!=null)
                    {
                        prev.next = cur.right;
                    }
                    else if(head == null)
                    {
                        head = cur.right;
                    }
                    prev = cur.right;
                }
                cur = cur.next;
            }
            cur = head;
            head = null;
            prev = null;
        }
    }
}