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;
}
};