Title

the problem link

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

算路径值，不过这个重要的是任意开始位置和任意推出位置，因此需要递归

Solution

//cpp:
class Solution {
public:
	int pathFrom(TreeNode* t, int sum) {
		if (t==NULL)
			return 0;
		return (sum == t->val?1:0) + pathFrom(t->left, sum - t->val) + pathFrom(t->right, sum - t->val);
	}
	int pathSum(TreeNode* root, int sum) {
		if (root==NULL) {
			return 0;
		}
		return pathFrom(root, sum) + pathSum(root->left, sum) + pathSum(root->right, sum);
	}
};