題目如下：

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

解題思路：題目數據量不大，我嘗試著用遞歸嵌套遞歸的方法，發現也能AC。

代碼如下：

# Definition for a binary tree node.
# class TreeNode(object):
 

#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    res = 0
    def recursive(self,node,sum):
        self.recursive2(node,0,sum)
        if node.left != None:
            self.recursive(node.left, sum)
        if node.right != None:
            self.recursive(node.right,sum)
    
 
def recursive2(self,node,amount,sum):
        if node.val + amount == sum:
            self.res += 1
        if node.left != None:
            self.recursive2(node.left, amount + node.val , sum)
        if node.right != None:
            self.recursive2(node.right,amount + node.val, sum)

    def pathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: int
        """
        self.res = 0
        if root != None:
            self.recursive(root,sum)
        return self.res
        

【leetcode】437. Path Sum III