```
var minDepth = function(root) {
if(root === null) return 0;
if(root.left === null) return 1 + minDepth(root.right);
if(root.right === null) return 1 + minDepth(root.left);
return Math.min(1 + minDepth(root.left), 1 + minDepth(root.right));
};
```