LeetCode---437. Path Sum III

題目

給出一個二叉樹和一個目標值值，找到所有的路徑，該路徑上的節點數值和爲目標值。路徑不一定非要用根節點開始。舉個例子：

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

Python題解

class Solution(object):
    def helper(self, root, target, so_far, cache):
        if root:
            complement = so_far + root.val - target
            if complement in cache:
                self.result += cache[complement]
            cache.setdefault(so_far + root.val, 0)
            cache[so_far + root.val] += 1
            self.helper(root.left, target, so_far + root.val, cache)
            self.helper(root.right, target, so_far + root.val, cache)
            cache[so_far + root.val] -= 1

    def pathSum(self, root, sum_val):
        """
        :type root: TreeNode
        :type sum_val: int
        :rtype: int
        """
        self.result = 0
        self.helper(root, sum_val, 0, {0: 1})
        return self.result