flatten-binary-tree-to-linked-list.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    TreeNode prev;
    public void flatten(TreeNode root) {
        if (root == null) return;
        
        flatten(root.right);
        flatten(root.left);
        
        root.right = prev;
        root.left = null;
        
        prev = root;
    }
    
   
}

/**

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


**/