Maximum Number of Integers to Choose From a Range I — C++

// Time:  O(b)
// Space: O(b)

// math
class Solution {
public:
    int maxCount(vector<int>& banned, int n, long long maxSum) {
        int k = min(static_cast<int>(((-1 + sqrt(1 + 8 * maxSum)) / 2)), n); // k = argmax((k+1)*k//2 <= maxSum)
        int64_t total = static_cast<int64_t>(k + 1) * k / 2;
        int result = k;
        unordered_set<int> lookup(cbegin(banned), cend(banned));
        for (const auto& x : lookup) {
            if (x <= k) {
                total -= x;
                --result;
            }
        }
        for (int i = k + 1; i <= n; ++i) {
            if (lookup.count(i)) {
                continue;
            }
            if (total + i > maxSum) {
                break;
            }
            total += i;
            ++result;
        }
        return result;
    }
};

// Time:  O(blogb + logn * logb)
// Space: O(b)
// binary search, prefix sum
class Solution2 {
public:
    int maxCount(vector<int>& banned, int n, long long maxSum) {
        unordered_set<int> banned_set(cbegin(banned), cend(banned));
        vector<int> sorted_banned(cbegin(banned_set), cend(banned_set));
        sort(begin(sorted_banned), end(sorted_banned));
        vector<int64_t> prefix(size(sorted_banned) + 1);
        for (int i = 0; i < size(sorted_banned); ++i) {
            prefix[i + 1] = prefix[i] + sorted_banned[i];
        }
        const auto& check = [&](const int64_t x) {
            return (x + 1) * x / 2 - prefix[distance(cbegin(sorted_banned), upper_bound(cbegin(sorted_banned), cend(sorted_banned), x))] <= maxSum;
        };
        int left = 1, right = n;
        while (left <= right) {
            const auto mid = left + (right - left) / 2;
            if (!check(mid)) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        } 
        return right - distance(cbegin(sorted_banned), upper_bound(cbegin(sorted_banned), cend(sorted_banned), right));
    }
};

// Time:  O(n)
// Space: O(b)
// greedy
class Solution3 {
public:
    int maxCount(vector<int>& banned, int n, int maxSum) {
        unordered_set<int> lookup(cbegin(banned), cend(banned));
        int result = 0;
        for (int i = 1, total = 0; i <= n; ++i) {
            if (lookup.count(i)) {
                continue;
            }
            if (total + i > maxSum) {
                break;
            }
            total += i;
            ++result;
        }
        return result;
    }
};