Minimum Depth of Binary Tree

class Solution {
public:
    void dfs(TreeNode *root,int &MIN,int step)
    {
         if(root==NULL) return ;
         if(root->left==NULL && root->right==NULL)
         {
             if(MIN>step) MIN=step;
             return ;
         }
         dfs(root->left,MIN,step+1);
         dfs(root->right,MIN,step+1);
    }
    int minDepth(TreeNode *root) {
        if(root==NULL) return 0;
        int MIN=9999999;
        dfs(root,MIN,1);
        return MIN;
    }
};