WEI-CHENG CHEN

解法：

class Solution {
public:
    vector<TreeNode*> v;
    void DFS(TreeNode* root){
        if (!root) return;
        v.push_back(root);
        DFS(root->left);
        DFS(root->right);
    }
    void flatten(TreeNode* root) {
        if (!root) return;
        DFS(root);
        for(int i=0;i<v.size()-1;i++){
            root->left = nullptr;
            root->right = v[i+1];
            root = root->right;
        }
    }
};