使用了前缀和

class Solution {
public:
    unordered_map<long long, long long> cnt;
    long long res=0;
    int pathSum(TreeNode* root, long long sum) {
        cnt[0]=1;
        dfs(root, sum, 0);
        return res;
    }
    void dfs(TreeNode* root, long long sum, long long cur)
    {
        if(!root)return;
        cur+=root->val;
        res+=cnt[cur-sum];
        cnt[cur]++;
        dfs(root->left, sum, cur);
        dfs(root->right, sum, cur);
        cnt[cur]--;
    }
};