112. 路径总和

https://leetcode-cn.com/problems/path-sum/
思路：dfs，每次用sum减去当前节点的值，直到sum为0

class Solution:
    def hasPathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: bool
        """
        #空节点
        if not root:
            return False
        sum -= root.val
        #sum为0，且是叶节点
        if not root.left and not root.right and sum == 0:
            return True
        #对左右子树递归
        return self.hasPathSum(root.left, sum) or self.hasPathSum(root.right, sum)