```
public class Solution {
public int minDepth(TreeNode root) {
if(root==null) return 0;
if(root.left==null) return minDepth(root.right)+1;
if(root.right==null) return minDepth(root.left)+1;
return Math.min(minDepth(root.left)+1,minDepth(root.right)+1);
}
}
```