Solution.cpp

/**
 * 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 pathSum(TreeNode* root, int targetSum) {
        unordered_map<long, int> cnt;
        cnt[0] = 1;
        function<int(TreeNode*, long)> dfs = [&](TreeNode* node, long s) -> int {
            if (!node) return 0;
            s += node->val;
            int ans = cnt[s - targetSum];
            ++cnt[s];
            ans += dfs(node->left, s) + dfs(node->right, s);
            --cnt[s];
            return ans;
        };
        return dfs(root, 0);
    }
};