The root of a binary tree and a value of a node (k) is given. The goal is to find distance of the closest leaf from that node.
For example if the tree is something like this:

					 7
					/ \
				   5   6
				  / \
			     2   3
			    / \  / \
		       9  11 8  14
	          /   / \ \   \
            10   12 15 16  18

if k=5 then answer will be 2 (node 6 is at distance 2 from node 5).
I'm stuck. Please help me how to approach it.