623. 在二叉树中增加一行

原题链接：623. 在二叉树中增加一行

solution：

/**
 * 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:
    int level;
    void dfs(TreeNode *root, int val, int depth) {
        if(root == nullptr) return;

        if(depth == level - 1) {
            root->left = new TreeNode(val, root->left, nullptr);
            root->right = new TreeNode(val, nullptr, root->right);
            return;
        }
        dfs(root->left, val, depth + 1);
        dfs(root->right, val, depth + 1);
    }


    TreeNode* addOneRow(TreeNode* root, int val, int depth) {
        if(depth == 1) {
            return new TreeNode(val, root, nullptr);
        }
        level = depth;
        dfs(root, val, 1);
        return root;
    }
};