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/**
 * 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 *next = NULL;
        dfs(root, next);
    }

    void dfs(TreeNode* &root, TreeNode* &next){
        if(!root) return;
        dfs(root->right, next);
        dfs(root->left, next);
        root->right = next;
        root->left = NULL;
        next = root;
    }
};