給定一個二叉樹，找出其最小深度。

最小深度是從根節點到最近葉子節點的最短路徑上的節點數量。

說明: 葉子節點是指沒有子節點的節點。

示例:

給定二叉樹 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回它的最小深度  2.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
 
class Solution:
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root == None:
            return 0
        elif root.left == None and root.right == None:
            return 1
        elif root.left == None and root.right != None:
            return self.minDepth(root.right)+1
        elif root.right ==None and root.left != None:
            return self.minDepth(root.left)+1
        elif root.left != None and root.right !=None:
            return min(self.minDepth(root.left), self.minDepth(root.right))+1