Given a binary tree, find its minimum depth.
Example 1:
Input:
1
/ \
3 2
/
4
Output: 2
Explanation:
Minimum depth is between nodes 1 and 2 since
minimum depth is defined as the number of
nodes along the shortest path from the root
node down to the nearest leaf node.
Example 2:
Input:
10
/ \
20 30
\ \
40 60
/
2
Output: 3
Explanation:
Minimum depth is between nodes 10,20 and 40.
Your Task:
You don't need to read input or print anything. Complete the function minDepth() which takes the root node as an input parameter and returns the minimum depth.

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(height of the tree)

Constraints:
1 ≤ N ≤ 10^5

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