进入pathSum函数后有两种情况,一种是包括root的值一种是不包括.

# 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 __init__(self):        self.ans=0            def pathSum(self, root, sum):        """        :type root: TreeNode        :type sum: int        :rtype: int        """        if not root:            return 0                self.pathSumWithNode(root,sum)        self.pathSum(root.left,sum)        self.pathSum(root.right,sum)                return self.ans    def pathSumWithNode(self,root,sum):        if not root:            return 0        if root.val==sum:            self.ans+=1        self.pathSumWithNode(root.left,sum-root.val)        self.pathSumWithNode(root.right,sum-root.val)