Given a binary tree, find its minimum depth.
Examples:
Input: root = [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.
Input: root = [10, 20, 30, N, 40, N, 60, N, N, 2]
Output: 3
Explanation: Minimum depth is between nodes 10,20 and 40.
Expected Time Complexity: O(n)
Expected Auxiliary Space: O(height of the tree)
Constraints:
1 ≤ number of nodes ≤ 105