简单递归.

# Definition for a binary tree node.# class TreeNode(object):#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution(object):    def hasPathSum(self, root, sum):        """        :type root: TreeNode        :type sum: int        :rtype: bool        """        if not root:            return False        if (not root.left) & (not root.right):            return root.val==sum        elif not root.left:            return self.hasPathSum(root.right,sum-root.val)        elif not root.right:            return self.hasPathSum(root.left,sum-root.val)        else:            return (self.hasPathSum(root.left,sum-root.val))|(self.hasPathSum(root.right,sum-root.val))