994. Rotting_Oranges.cpp

class Solution {
 public:
  int orangesRotting(vector<vector<int>>& grid) {
    const int m = grid.size();
    const int n = grid[0].size();
    const vector<int> dirs{0, 1, 0, -1, 0};

    auto isNeighborRotten = [&](int i, int j, const vector<vector<int>>& grid) {
      for (int k = 0; k < 4; ++k) {
        const int r = i + dirs[k];
        const int c = j + dirs[k + 1];
        if (r < 0 || r == m || c < 0 || c == n)
          continue;
        if (grid[r][c] == 2)
          return true;
      }
      return false;
    };

    int ans = 0;

    while (true) {
      vector<vector<int>> nextGrid(m, vector<int>(n));
      // Calculate `nextGrid` based on `grid`
      for (int i = 0; i < m; ++i)
        for (int j = 0; j < n; ++j)
          if (grid[i][j] == 1) {  // Fresh
            if (isNeighborRotten(
                    i, j, grid))  // Any of 4-directionally oranges is rotten
              nextGrid[i][j] = 2;
            else
              nextGrid[i][j] = 1;
          } else if (grid[i][j] == 2) {  // Rotten
            nextGrid[i][j] = 2;          // Keep rotten
          }
      if (nextGrid == grid)
        break;
      grid = nextGrid;
      ++ans;
    }

    return any_of(
               begin(grid), end(grid),
               [&](vector<int>& row) {
      return any_of(begin(row), end(row),
                    [&](int orange) { return orange == 1; });
               })
        ? -1
        : ans;
  }
};