LeetCode Minimum Depth of Binary Tree

class Solution {
public:
    int minDepth(TreeNode *root) {
        if (root == NULL) return 0;
        int min_depth = INT_MAX;
        dfs(root, 0, min_depth);
        return min_depth + 1;
    }
    
    void dfs(TreeNode *root, int cur_depth, int& min_depth) {
        if (root == NULL) return;
        if (cur_depth >= min_depth) return;
        // we come here means that cur_depth < min_depth
        if (root->left == NULL && root->right == NULL) { // this is a leaf node
            min_depth = cur_depth;
            return;
        }
        
        dfs(root->left, cur_depth + 1, min_depth);
        dfs(root->right, cur_depth + 1, min_depth);
    }
};