Maximum Number of Groups With Increasing Length — C++

// Time:  O(n)
// Space: O(n)

// constructive algorithms, counting sort, greedy
class Solution {
public:
    int maxIncreasingGroups(vector<int>& usageLimits) {
        const auto& inplace_counting_sort = [](vector<int> *nums, bool is_reverse) {
            if (empty(*nums)) {
                return;
            }
            const int max_num = *max_element(cbegin(*nums), cend(*nums));
            vector<int> count(max_num + 1);
            for (const auto& num : *nums) {
                ++count[num];
            }
            for (int i = 1; i < size(count); ++i) {
                count[i] += count[i - 1];
            }
            for (int i = size(*nums) - 1; i >= 0; --i) {  // inplace but unstable sort
                while ((*nums)[i] >= 0) {
                    --count[(*nums)[i]];
                    const int j = count[(*nums)[i]];
                    tie((*nums)[i], (*nums)[j]) = pair((*nums)[j], ~(*nums)[i]);
                }
            }
            for (auto& num : *nums) {
                num = ~num;  // restore values
            }
            if (is_reverse) {  // unstable sort
                reverse(begin(*nums), end(*nums));
            }
        };
        
        for (auto& x : usageLimits) {
            x = min(x, static_cast<int>(size(usageLimits)));
        }
        inplace_counting_sort(&usageLimits, false);
        int result = 0;
        int64_t curr = 0;
        for (const auto& x : usageLimits) {
            curr += x;
            if (curr >= result + 1) {
                curr -= result + 1;
                ++result;
            }
        }
        return result;
    }
};

// Time:  O(nlogn)
// Space: O(1)
// constructive algorithms, sort, greedy
class Solution2 {
public:
    int maxIncreasingGroups(vector<int>& usageLimits) {
        sort(begin(usageLimits), end(usageLimits));
        int result = 0;
        int64_t curr = 0;
        for (const auto& x : usageLimits) {
            curr += x;
            if (curr >= result + 1) {
                curr -= result + 1;
                ++result;
            }
        }
        return result;
    }
};

// Time:  O(nlogn)
// Space: O(1)
// constructive algorithms, sort, binary search, greedy
class Solution3 {
public:
    int maxIncreasingGroups(vector<int>& usageLimits) {
        const auto& check = [&](int l) {
            int64_t curr = 0;
            for (int i = 0; i < l; ++i) {
                curr += usageLimits[(size(usageLimits) - 1) - i] - (l - i);
                curr = min(curr, static_cast<int64_t>(0));
            }
            for (int i = 0; i < size(usageLimits) - l; ++i) {
                curr += usageLimits[i];
            }
            return curr >= 0;
        };

        sort(begin(usageLimits), end(usageLimits));
        int left = 1, right = size(usageLimits);
        while (left <= right) {
            const int mid = left + (right - left) / 2;
            if (!check(mid)) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }
        return right;
    }
};


// Time:  O(nlogn)
// Space: O(n)
// constructive algorithms, sort, binary search, greedy, prefix sum
class Solution4 {
public:
    int maxIncreasingGroups(vector<int>& usageLimits) {
        sort(begin(usageLimits), end(usageLimits));
        vector<int64_t> prefix(size(usageLimits) + 1);
        for (int i = 0; i < size(usageLimits); ++i) {
            prefix[i + 1] = prefix[i] + usageLimits[i];
        }
        const auto& check = [&](int l) {
            for (int i = 1; i <= l; ++i) {
                if (static_cast<int64_t>(i + 1) * i / 2 > prefix[size(usageLimits) - (l - i)]) {
                    return false;
                }
            }
            return true;
        };

        int left = 1, right = size(usageLimits);
        while (left <= right) {
            const int mid = left + (right - left) / 2;
            if (!check(mid)) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }
        return right;
    }
};