Probability of a Two Boxes Having The Same Number of Distinct Balls — C++

// Time:  O(k^3 * n^2)
// Space: O(k^2 * n)

class Solution {
public:
    double getProbability(vector<int>& balls) {
        unordered_map<pair<int, int>, uint64_t, PairHash<int>> dp;
        dp[pair(0, 0)] = 1;  // dp[i, j] is the ways of number difference i and color difference j
        for (const auto& n : balls) {  // O(k) times
            unordered_map<pair<int, int>, uint64_t, PairHash<int>> new_dp;
            for (const auto& kvp : dp) {  // O(k^2 * n) times
                const auto& [ndiff, cdiff] = kvp.first;
                for (int k = 0, new_count = 1; k <= n; ++k, new_count *= n - k + 1, new_count /= k) {  // O(n) times
                    const auto& new_ndiff = ndiff + (k - (n - k));
                    const auto& new_cdiff = (k == 0) ? cdiff - 1 : ((k == n) ? cdiff + 1 : cdiff);
                    new_dp[pair(new_ndiff, new_cdiff)] += kvp.second * new_count;
                }
            }
            dp = move(new_dp);
        }
        const auto& total = accumulate(cbegin(balls), cend(balls), 0);
        return double(dp[pair(0, 0)]) / nCr(total, total / 2);
    }

private:
    uint64_t nCr(int n, int r) {  // Time: O(n), Space: O(1)
        if (n - r < r) {
            return nCr(n, n - r);
        }
        uint64_t c = 1;
        for (int k = 1; k <= r; ++k) {
            c *= n - k + 1;
            c /= k;
        }
        return c;
    }

    template <typename T>
    struct PairHash {
        size_t operator()(const pair<T, T>& p) const {
            size_t seed = 0;
            seed ^= std::hash<T>{}(p.first)  + 0x9e3779b9 + (seed<<6) + (seed>>2);
            seed ^= std::hash<T>{}(p.second) + 0x9e3779b9 + (seed<<6) + (seed>>2);
            return seed;
        }
    };
};