LeetCode-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

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

Solution:

Code:

<span style="font-size:14px;">/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode * helper(TreeNode *root) {        if (!root) return root;        if (!root->left && !root->right) return root;        TreeNode *leftEnd = helper(root->left);        TreeNode *rightEnd = helper(root->right);        TreeNode *right = root->right;        if (root->left) {            root->right = root->left;            root->left = NULL;            leftEnd->right = right;        }        if (right) return rightEnd;        else return leftEnd;    }    void flatten(TreeNode *root) {        if (!root) return;        helper(root);    }};</span>