Based on: https://www.geeksforgeeks.org/find-minimum-depth-of-a-binary-tree/amp/

Given a binary tree, find its minimum depth. The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

Given the below Binary Tree:

Minimum height of below Binary Tree is 2. Note that the path must end on a leaf node.

• traverse the given Binary Tree and for every node, check if it is a leaf node
• If yes, then return 1
• If not leaf node then if the left subtree is NULL, then recur for the right subtree
• if the right subtree is NULL, then recur for the left subtree
• if both left and right subtrees are not NULL, then take the minimum of two heights.