Leetcode 437. Path Sum III

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)