LeetCode/problems/max-sum-of-rectangle-no-larger-than-k.cpp

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#include <set>
#include <vector>
class Solution {
void row_partial_sums(const std::vector<std::vector<int>>& matrix, std::size_t y, std::vector<int>& partial) const
{
for (std::size_t x = 0; x < partial.size(); x++) {
partial[x] += matrix[y][x];
}
}
int find(std::vector<int>& partial_sums, int k) const
{
int max_sum = INT_MIN;
std::set<int> prefixes;
prefixes.insert(0);
int running_sum = 0;
for (auto partial : partial_sums) {
running_sum += partial;
auto found = prefixes.lower_bound(running_sum - k);
if (found != prefixes.end()) {
max_sum = std::max(max_sum, running_sum - *found);
}
prefixes.insert(running_sum);
}
return max_sum;
}
public:
int maxSumSubmatrix(const std::vector<std::vector<int>>& matrix, int k) const
{
std::size_t rows = matrix.size();
std::size_t cols = matrix.front().size();
int max_sum = INT_MIN;
for (std::size_t y_min = 0; y_min < rows; y_min++) {
std::vector<int> partial_sums(cols, 0);
for (std::size_t y_max = y_min; y_max < rows; y_max++) {
row_partial_sums(matrix, y_max, partial_sums);
int maximum = find(partial_sums, k);
max_sum = std::max(max_sum, maximum);
}
}
return max_sum;
}
};