leetcode623+给tree加一层值，递归

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https://leetcode.com/problems/add-one-row-to-tree/description/

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
public:
    void dfs(TreeNode* r, int cur_d, int v, int d)
    {
        if(r==NULL) return;
        if(cur_d == d-1){
            TreeNode *new_left = new TreeNode(v);
            TreeNode *new_right = new TreeNode(v);
            new_left->left = r->left;
            new_right->right = r->right;
            r->left = new_left;
            r->right = new_right;
        }
        dfs(r->left, cur_d+1, v, d);
        dfs(r->right, cur_d+1, v, d);
    }
    TreeNode* addOneRow(TreeNode* root, int v, int d) {
        if(d==1){
            TreeNode *new_root = new TreeNode(v);
            new_root->left = root;
            return new_root;
        }
        dfs(root, 1, v, d);
        return root;
    }
};