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.

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

class Solution(object):
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0
        left = self.minDepth(root.left)
        right = self.minDepth(root.right)
        if not left and not right:
            return 1
        elif not left:
            return right + 1
        elif not right:
            return left + 1
        else:
            return min(left, right) + 1