2020 August LeetCoding Challenge - Week 2

LeetCode 八月挑战专题每日随缘更新,点击类别 2020 August LeetCoding Challenge 查看更多。

August 8th: 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

题目解析

使用函数 func 对树进行深度优先搜索,对以 node 为起点且满足题目要求的路径进行计数。

pathSum(root->left, sum)pathSum(root->right, sum) 则是分别给左右节点一次「重新开始」的机会。

C++ 实现

class Solution {
public:
int pathSum(TreeNode* root, int sum) {
if(!root) return 0;
return func(root, sum) + pathSum(root->left, sum) + pathSum(root->right, sum);
}
int func(TreeNode *node,int sum){
if(!node) return 0;
int ans = node->val==sum ? 1 : 0;
return ans + func(node->left, sum-node->val) + func(node->right, sum-node->val);
}
};

终究还是没能在工作日坚持下来,这个系列终止吧。