Bug Report for design-twitter-feed

[raymowu]

Bug Report for https://neetcode.io/problems/design-twitter-feed

Please describe the bug below and include any steps to reproduce the bug or screenshots if possible.

This c++ solution is incorrect:
`class Twitter {
public:
int count;
unordered_map<int, vector<pair<int,int>>> tweetMap;
unordered_map<int, unordered_set> followMap;

Twitter() {
    count = 0;
}

void postTweet(int userId, int tweetId) {
    tweetMap[userId].push_back({count, tweetId});
    if (tweetMap[userId].size() > 10) {
        tweetMap[userId].erase(tweetMap[userId].begin());
    }
    count--;
}

vector<int> getNewsFeed(int userId) {
    vector<int> res;
    followMap[userId].insert(userId);
    priority_queue<vector<int>, vector<vector<int>>, greater<vector<int>>> minHeap;
    if (followMap[userId].size() >= 10) {
        priority_queue<vector<int>> maxHeap;
        for (auto f : followMap[userId]) {
            if (!tweetMap.count(f)) continue;
            int idx = tweetMap[f].size() - 1;
            auto &p = tweetMap[f][idx];
            maxHeap.push({-p.first, p.second, f, idx - 1});
            if (maxHeap.size() > 10) maxHeap.pop();
        }
        while (!maxHeap.empty()) {
            auto t = maxHeap.top();
            maxHeap.pop();
            minHeap.push({-t[0], t[1], t[2], t[3]});
        }
    } else {
        for (auto f : followMap[userId]) {
            if (!tweetMap.count(f)) continue;
            int idx = tweetMap[f].size() - 1;
            auto &p = tweetMap[f][idx];
            minHeap.push({p.first, p.second, f, idx - 1});
        }
    }
    while (!minHeap.empty() && res.size() < 10) {
        auto t = minHeap.top();
        minHeap.pop();
        res.push_back(t[1]);
        int idx = t[3];
        if (idx >= 0) {
            auto &p = tweetMap[t[2]][idx];
            minHeap.push({p.first, p.second, t[2], idx - 1});
        }
    }
    return res;
}

void follow(int followerId, int followeeId) {
    followMap[followerId].insert(followeeId);
}

void unfollow(int followerId, int followeeId) {
    if (followMap[followerId].count(followeeId)) {
        followMap[followerId].erase(followeeId);
    }
}

};`

line 29 should be p.first not -p.first and line 35 should be t[0] not -t[0]. Invariant is that smaller timestamps are newer, so if converting to positive, that makes the maxheap push out the newer tweets instead of older tweets.