二叉树最小深度该怎么写呢

请教下面图片中的这个内容该怎么做呢，我能该如何去解决这种问题呢？

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
private:
    int next_layer_num=0;
    int current_layer_num=1;
    int deep_min=1;
    queue <struct TreeNode*> q;
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    int run(TreeNode* root) {
        if(root == NULL) return 0;
        q.push(root);
        do{
            struct TreeNode * node = q.front();
            if(node->left){
                ++next_layer_num;
                q.push(node->left);
            }
            if(node->right){
                ++next_layer_num;
                q.push(node->right);
            }
            if(node->left==NULL && node->right==NULL)
                return deep_min;

            q.pop();
            if(--current_layer_num==0)
            {
                current_layer_num=next_layer_num;
                next_layer_num=0;
                ++deep_min;
            }
        }while(!q.empty());
        return 0;
    }
};