Example: 4 / \ 2 8 / \ 5 10 Output ==> 2
1. If the root node is null, return null.
root
null
return null
2. Else, if the left node of the root node is null, return root
3. Recurse through the left node. i.e. return findMin(root.left)
left
findMin(root.left)
public TreeNode findMin(TreeNode root) { }
