[Leetcode] 114. Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

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