Multi-source BFS (Graph BFS / DFS Sub-pattern)

// Multi-source BFS: every source goes in the queue at distance 0, then all spread together.
vector<vector<int>> nearestSource(const vector<vector<int>>& grid) {   // 1 = source, 0 = open, -1 = wall
    int R = grid.size(), C = grid[0].size();
    vector<vector<int>> dist(R, vector<int>(C, -1));                     // -1 = not reached yet
    queue<pair<int, int>> q;
    for (int r = 0; r < R; r++)
        for (int c = 0; c < C; c++)
            if (grid[r][c] == 1) { dist[r][c] = 0; q.push({r, c}); }
    int dr[4] = {1, -1, 0, 0}, dc[4] = {0, 0, 1, -1};
    while (!q.empty()) {
        auto [r, c] = q.front(); q.pop();
        for (int d = 0; d < 4; d++) {
            int nr = r + dr[d], nc = c + dc[d];
            if (nr < 0 || nc < 0 || nr >= R || nc >= C || grid[nr][nc] == -1 || dist[nr][nc] != -1) continue;
            dist[nr][nc] = dist[r][c] + 1;        // first time reached = nearest source
            q.push({nr, nc});
        }
    }
    return dist;
}