/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void flatten(TreeNode* root) {
TreeNode*last = NULL;
preorder(root,last);
}
void preorder(TreeNode* node,TreeNode* &last){
if(!node) return;
if( !node->right &&!node->left){
last = node;
return;
}TreeNode* left = node->left;
TreeNode* right = node->right;
TreeNode* left_last = NULL;
TreeNode* right_last = NULL;
if(left){
preorder(left,left_last);
node->left = NULL;
node->right = left;
last = left_last;
}
if(right){
preorder(right,right_last);
if(left_last){
left_last->right = right;
}
last = right_last;
}
}
};