437. Path Sum III

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int ans;
    
    void f(TreeNode* root, int targetSum, long long prev) {
        if (!root) return;
        long long cur = prev + root->val;
        ans += targetSum == cur;
        
        f(root->left, targetSum, cur);
        f(root->right, targetSum, cur);
    }
    
    int pathSum(TreeNode* root, int targetSum) {
        f(root, targetSum, 0);
        
        if (!root) return 0;
        pathSum(root->left, targetSum);
        pathSum(root->right, targetSum);
        
        return ans;
    }
};