```
class Solution {
public:
int minDepth(TreeNode* root) {
if(root==NULL)return 0;
else if(root->left==NULL&&root->right==NULL)return 1;
else if(root->left==NULL)return minDepth(root->right)+1;
else if(root->right==NULL)return minDepth(root->left)+1;
else return min(minDepth(root->left),minDepth(root->right))+1;
}
};
```