LeetCode 437. Path Sum III
2016-11-12 11:24
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Problem Statement
(Source) 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
Tags:
Tree.
Solution
Idea:TODO.
# 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): def pathSum(self, root, sum): """ :type root: TreeNode :type sum: int :rtype: int """ prefix_sum = {0 : 1} return self.helper(root, sum, prefix_sum, 0) def helper(self, root, target, prefix_sum, cur): if not root: return 0 total = cur + root.val res = prefix_sum.get(total - target, 0) prefix_sum[total] = prefix_sum.get(total, 0) + 1 res += self.helper(root.left, target, prefix_sum, total) + \ self.helper(root.right, target, prefix_sum, total) prefix_sum[total] -= 1 return res
Complexity Analysis:
Time Complexity: O(n).
Space Complexity: ?.
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