Get Biggest Three Rhombus Sums in a Grid — C++

// Time:  O(m * n * min(m, n))
// Space: O(m * n)

class Solution {
public:
    vector<int> getBiggestThree(vector<vector<int>>& grid) {
        static const int K = 3;

        vector<vector<int>> left{grid}, right{grid};
        for (int i = 1; i < size(grid); ++i) {
            for (int j = 0; j < size(grid[0]) - 1; ++j) {
                left[i][j] += left[i - 1][j + 1];
            }
        }
        for (int i = 1; i < size(grid); ++i) {
            for (int j = 1; j < size(grid[0]); ++j) {
                right[i][j] += right[i - 1][j - 1];
            }
        }
        priority_queue<int, vector<int>, greater<int>> min_heap;
        unordered_set<int> lookup;
        for (int k = 0; k <= (min(size(grid), size(grid[0])) + 1) / 2; ++k) {
            for (int i = k; i < size(grid) - k; ++i) {
                for (int j = k; j < size(grid[0]) - k; ++j) {
                    int total = k ? ((left[i][j - k] - left[i - k][j]) + (right[i][j + k] - right[i - k][j]) + grid[i - k][j]) +  
                                    ((left[i + k][j] - left[i][j + k]) + (right[i + k][j] - right[i][j - k]) - grid[i + k][j])
                                  : grid[i][j];
                    if (lookup.count(total)) {
                        continue;
                    }
                    lookup.emplace(total);
                    min_heap.emplace(total);
                    if (size(min_heap) == K + 1) {
                        lookup.erase(min_heap.top()); min_heap.pop();
                    }
                }
            }
        }
        vector<int> result;
        while (!empty(min_heap)) {
            result.emplace_back(min_heap.top()); min_heap.pop();
        }
        reverse(begin(result), end(result));
        return result;
    }
};