Get Biggest Three Rhombus Sums in a Grid LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> getBiggestThree(vector<vector<int>> &grid) {
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    int n = grid.size(), m = grid[0].size();
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    set<int> s;
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    // 1x1, 3x3, 5x5
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    for (int len = 1; len <= min(m, n); len += 2) {
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      for (int i = 0; i + len <= n; i++) {
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        for (int j = 0; j + len <= m; j++) {
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          int d = len / 2;
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          if (d == 0) {
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            s.insert(grid[i][j]);
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          } else {
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            int x = i, y = j + d;
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            long long sum = 0;
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            for (int k = 0; k < d; k++) sum += grid[x++][y++];
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            for (int k = 0; k < d; k++) sum += grid[x++][y--];
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            for (int k = 0; k < d; k++) sum += grid[x--][y--];
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            for (int k = 0; k < d; k++) sum += grid[x--][y++];
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            s.insert(sum);
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          }
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        }
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      }
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    }
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    if (s.size() < 3) return vector<int>(s.rbegin(), s.rend());
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    return vector<int>(s.rbegin(), next(s.rbegin(), 3));
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  }
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};