```
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),minDepth(root.right))+1;
}
}
```