[leetcode] 437. Path Sum III 解题报告

题目链接: https://leetcode.com/problems/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

思路: 从每一个节点单独开始记录路径的和, 最后结果即为从当前节点开始的路径的结果个数和从左右节点开始计数的结果个数.

代码如下:

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int DFS(TreeNode* root, int sum){
if(!root) return 0;
sum -= root->val;
return !sum + DFS(root->left, sum) + DFS(root->right, sum);
}

int pathSum(TreeNode* root, int sum) {
if(!root) return 0;
int ans = pathSum(root->left, sum) + pathSum(root->right, sum);
return DFS(root, sum) + ans;
}
};