左右子树只存在一个时不能返回0, 而是要沿着那个子树继续找下去。

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return an integer
    def minDepth(self, root):
        if root == None: 
            return 0
        if root.left == None and root.right == None:
            return 1
        if root.left == None:
            return self.minDepth(root.right) + 1
        if root.right == None:
            return self.minDepth(root.left) + 1
        return min(self.minDepth(root.left), self.minDepth(root.right)) + 1