45: Minimum depth of a binary tree

Given a binary tree. Find a minimum depth of the binary tree

Note: The minimum depth is the number of nodes from the root node to the nearest leaf node

 

Example 1

Input:        1
             / \
            2   3
               / \
               6  7

Output: 2

Explanation: 1 -> 2

Example 2

Input:     1   
            \  
             3 

Output: 2