You are given a binary tree in which each node contains an integer value.
Find the number of paths that sum to a given value.
The path does not need to start or end at the root or a leaf, but it must go downwards (traveling only from parent nodes to child nodes).
The tree has no more than 1,000 nodes and the values are in the range -1,000,000 to 1,000,000.
Example:
root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8
10
/ \
5 -3
/ \ \
3 2 11
/ \ \
3 -2 1
Return 3. The paths that sum to 8 are:
1. 5 -> 3
2. 5 -> 2 -> 1
3. -3 -> 11
本题的思路是写一个辅助函数helper(node)来return以node为起点的和为sum的path的条数。然后对所有的节点，遍历一下helper就可以了。
class Solution(object):
def pathSum(self, root, sum):
"""
:type root: TreeNode
:type sum: int
:rtype: int
"""
if not root:
return 0
return self.helper(root, sum) + self.pathSum(root.left, sum) + self.pathSum(root.right, sum) def pathSumFromRoot(self, node, sum):
if not node:
return 0
if node.val == sum:
return 1 + self.helper(node.left, sum-node.val) + self.helper(node.right, sum-node.val)
else:
return self.helper(node.left, sum-node.val) + self.helper(node.right, sum-node.val)