885.cpp

class Solution {
  // Runtime: 52 ms, faster than 99.27% of C++ online submissions for Spiral
  // Matrix III. Memory Usage: 13.6 MB, less than 60.22% of C++ online
  // submissions for Spiral Matrix III.
public:
  vector<vector<int>> spiralMatrixIII(int R, int C, int r, int c) {
    vector<vector<int>> res = {{r, c}};
    vector<pair<int, int>> dirs = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};
    int curDir = 0;
    for (int n = 0; res.size() < R * C; ++n) {
      auto [dirX, dirY] = dirs[curDir++ % 4];
      for (int i = 0; i < n / 2 + 1; ++i) {
        r += dirY, c += dirX;
        if (0 <= r && r < R && 0 <= c && c < C)
          res.push_back({r, c});
      }
    }
    return res;
  }
};