437. Path Sum III

题目描述【Leetcode】

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:

5 -> 3

5 -> 2 -> 1

-3 -> 11

求满足路径和为sum的路径条数：

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

class Solution {
public:
int pathSum(TreeNode* root, int sum) {
int count = 0;
if(!root) return 0;
count = count + f(root,sum)+ pathSum(root->left,sum)+pathSum(root->right,sum);
return count;
}
};