Snail

• Time : $O(nm)$
• Space : $O(nm)$

#include <vector>
using namespace std;
std::vector<int> snail(const std::vector<std::vector<int>> &snail_map) {
    int n = snail_map.size(), m = snail_map[0].size();
  if(!n or !m) return {};
    vector<vector<bool>> vis(n, vector<bool>(m));
    vector<int> res;
    int dy[4]{0,1,0,-1}, dx[4]{1,0,-1,0}, d = 0, y = 0, x = 0;
    auto go = [&]() {
        for(int i = 0; i < 4; i++) {
            int ny = y + dy[d], nx = x + dx[d];
            if(0 <= ny and ny < n and 0 <= nx and nx < m and !vis[ny][nx]) {
                y = ny, x = nx;
                return true;
            }
            d = (d + 1) % 4;
        }
        return false;
    };
    do {
        res.push_back(snail_map[y][x]);
        vis[y][x] = true;
    } while(go());
    return res;
}