【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.

class Solution(object):

    def pathSum(self, root, sum):

        """

        :type root: TreeNode

        :type sum: int

        :rtype: int

        """

        if root:

            return self.findPath(root, sum)+self.pathSum(root.left, sum)+self.pathSum(root.right, sum)

        return 0

    def findPath(self, root, target):

        if root:

            return int(root.val==target)+self.findPath(root.left, target-root.val)+self.findPath(root.right, target-root.val)

        return 0

1 基于每个节点traverse，找到其所有的paths to find if there is sum to target

2 root value 等于target值也是一种情况

3 从每一个节点开始往左右子节点走，使用recursive的方法，target-->target-root.val-->...直到target==当前节点value，就相当于找到了一个path，返回1；如果节点不存在了，就返回0；当走到某一个节点时，target==当前节点value，这时会返回1，但是此node的左右节点都还存在，还是会继续往下走的，直到走到叶子节点

4 所有的path，要包括以root为根节点开始的+root.left为根节点开始的+root.right为根节点开始的