Given a binary tree, write a program to 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.
- A leaf is a node with no children.
Input: Given a binary tree [10,5,15,null,null,11,6] 10 / \ 5 15 / \ 11 6 Output: 2
Input: Given a binary tree [1,null,2] 1 \ 2 Output: 2