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

Flatten Binary Tree to Linked List.cpp

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(root == NULL) {
            return;
        } else if(root->left == NULL and root->right == NULL) {
            return;
        } else {
            flatten(root->left);
            flatten(root->right);
            
            if(root->left != NULL) {
                TreeNode* last = root->left;
                while(last->right != NULL) {
                    last = last->right;
                }
                
                last->right = root->right;
                root->right = root->left;
                root->left = NULL;
            }
        }
    }
};