Question

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

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

Solution

class Solution {
    TreeNode pre = null;
    public void flatten(TreeNode root) {
        if(root == null) return;
        flatten(root.right);
        flatten(root.left);
        root.right = pre;
        root.left = null;
        pre = root;        
    }
}