https://leetcode.com/problems/path-sum-iii/

1. Use recusion:

  1. //28 ms    15.7 MB
  3. /**
  4.  * Definition for a binary tree node.
  5.  * struct TreeNode {
  6.  *     int val;
  7.  *     TreeNode *left;
  8.  *     TreeNode *right;
  9.  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
  10.  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
  11.  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
  12.  * };
  13.  */
  14. class Solution {
  15. public:
  16.     //sum of paths
  17.     int pathSum(TreeNode* root, int sum) {
  18.         if (!root) return 0;
  20.         return numberOfPaths(root, sum) + 
  21.             pathSum(root->left, sum) + 
  22.             pathSum(root->right, sum);
  23.     }
  25. private:
  26.     //number of paths origniated by root
  27.     int numberOfPaths(TreeNode* root, int sum) {
  28.         if (!root) return 0;    
  29.         sum -= root->val;
  31.         if(sum == 0)
  32.             return 1 + numberOfPaths(root->left, sum) + numberOfPaths(root->right, sum);
  34.         return numberOfPaths(root->left, sum) + numberOfPaths(root->right, sum);
  35.   }
  36. };