input: Write a python function to find the minimum depth of a binary tree.
  assert minDepth(None) == 0
  assert minDepth(TreeNode(1)) == 1
  assert minDepth(TreeNode(1, TreeNode(2), TreeNode(3))) == 2
  assert minDepth(TreeNode(1, TreeNode(2, TreeNode(4)), TreeNode(3))) == 2
output: def minDepth(root):
      if not root:
          return 0
      if not root.left and not root.right:
          return 1
      if not root.left:
          return minDepth(root.right) + 1
      if not root.right:
          return minDepth(root.left) + 1
      return min(minDepth(root.left), minDepth(root.right)) + 1