Grid shortest path (Graph BFS / DFS Sub-pattern)

// Shortest path in an unweighted grid: BFS from the start; the first time we pop the target is the answer.
int shortestPath(const vector<vector<int>>& grid, pair<int, int> start, pair<int, int> target) {  // 0 = open, 1 = wall
    int R = grid.size(), C = grid[0].size();
    if (grid[start.first][start.second] || grid[target.first][target.second]) return -1;
    vector<vector<int>> dist(R, vector<int>(C, -1));
    queue<pair<int, int>> q;
    dist[start.first][start.second] = 0;
    q.push(start);
    int dr[4] = {1, -1, 0, 0}, dc[4] = {0, 0, 1, -1};
    while (!q.empty()) {
        auto [r, c] = q.front(); q.pop();
        if (make_pair(r, c) == target) return dist[r][c];
        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] || dist[nr][nc] != -1) continue;
            dist[nr][nc] = dist[r][c] + 1;        // mark when PUSHED, not when popped
            q.push({nr, nc});
        }
    }
    return -1;
}