437. 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

计算有多少条路径上的节点值的和为sum

rootSum是计算从根节点往下，有没有符合条件的子路径

C++(19ms):

/**
 * 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 pathSum(TreeNode* root, int sum) {
        if (root == NULL) return 0 ;
        return rootSum(root,sum) + pathSum(root->left,sum) + pathSum(root->right,sum) ;
    }
    
    int rootSum(TreeNode* root, int sum) {
        if (root == NULL) return 0 ;
        return (root->val == sum ? 1 : 0) + rootSum(root->left,sum - root->val) + rootSum(root->right,sum - root->val) ;
    }
};