Minimum_Depth_Of_Binary_Tree.py

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.

Note: A leaf is a node with no children.

Example 1:
Input: root = [3,9,20,null,null,15,7]
Output: 2

Example 2:
Input: root = [2,null,3,null,4,null,5,null,6]
Output: 5
 
Constraints:
The number of nodes in the tree is in the range [0, 105].
-1000 <= Node.val <= 1000

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right


# My Solution
class Solution:
    def minDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        if root.left is None and root.right is None:
            return 1
        elif root.left is None:
            return 1 + self.minDepth(root.right)
        elif root.right is None:
            return 1 + self.minDepth(root.left)
        else:
            return 1 + min(self.minDepth(root.left), self.minDepth(root.right))