Flatten Binary Tree to Linked List

Desicription

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

For example,
Given

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

The flattened tree should look like:

1
2
3
4
5
6
7
8
9
10
11
1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

Solution

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    TreeNode* prev = NULL;
public:
    void flatten(TreeNode* root) {
        if(root == NULL)
            return ;
        flatten(root->right);
        flatten(root->left);
        root->right = prev;
        root->left = NULL;
        prev = root;
    }
};