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.
/**
* 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 {
public:
void flatten(TreeNode* root) {
if (root == NULL) return;
TreeNode* temp = root->left;//左子树
if(temp != NULL){
while(temp->right != NULL){//左子树的最右节点
temp = temp->right;
}
temp->right = root->right;//右子树接到左子树的最右节点的右孩子
root->right = root->left;//转移左子树到右子树
root->left = NULL;//左子树置空
}
flatten(root->right);//递归处理右接孩子
}
};