速度比全排序或者heap/pq快到不知道哪里去了～

Time complexity: O(m*n)
Space complexity: O(m*n)

不用黑科技就达到99.77%

class Solution {
public:
  long long maxSum(vector<vector<int>>& grid, vector<int>& limits, int k) {
    // Step 1: Sort each row using fast parition, collect largest limits[i] elements.
    vector<int> nums;
    nums.reserve(grid.size() * grid[0].size());
    for (int i = 0; i < grid.size(); ++i) {
      nth_element(begin(grid[i]), begin(grid[i]) + limits[i], end(grid[i]), std::greater{});
      nums.insert(nums.end(), begin(grid[i]), begin(grid[i]) + limits[i]);
    }

    // Setp 2: fast partition to find the largest k elements. O(m*n).
    nth_element(begin(nums), begin(nums) + k, end(nums), std::greater{});
    
    // Step 3: Sum them up. O(k)
    return accumulate(begin(nums), begin(nums) + k, 0LL);
  }
};

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