The subtasks in this problem motivate starting by solving the problem when $K=1$ and then going from there to solving the problem when $K=2$ and then $K=3$. As a result, this editorial will go through solving each of these in order.

$K = 1$: If Bessie can only turn once, she must turn at either the top-right corner or the bottom-left corner. Therefore, it suffices to check that the top row and right column are empty and that the bottom row and left column are empty.

$K = 2$: If Bessie is to make exactly two turns, then either she walks along the top row, turns right and walks all the way to the bottom and then turns left, or she walks along the left column, turns left, and walks all the way to the right and then turns right. In the former case, we can brute force all columns Bessie would select. In the latter case, we can brute force all rows Bessie would select.

$K = 3$: If Bessie is to make exactly three turns, then Bessie ends up turning in the middle of the grid in some square that is not in the top row, bottom row, left column, or right column. We can brute force all inner squares that Bessie would select.

The runtime for a single test case is $\mathcal{O}(N^3)$ - there are $\mathcal{O}(N^2)$ paths that Bessie can consider, and there are $\mathcal{O}(N)$ squares on each path to validate as being empty.

#include <iostream> #include <string> #include <vector> using namespace std; void solve() { int n, k; cin >> n >> k; vector<string> g(n); for(int i = 0; i < n; i++) cin >> g[i]; int ret = 0; if(k >= 1) { bool urcorner = true; bool dlcorner = true; for(int i = 0; i < n; i++) { if(g[0][i] == 'H' || g[i][n-1] == 'H') urcorner = false; if(g[i][0] == 'H' || g[n-1][i] == 'H') dlcorner = false; } ret += urcorner; ret += dlcorner; } if(k >= 2) { // use column j for(int j = 1; j < n-1; j++) { bool valid = true; for(int i = 0; i < n; i++) { if(g[i][j] == 'H') valid = false; if(i < j && g[0][i] == 'H') valid = false; if(i > j && g[n-1][i] == 'H') valid = false; } ret += valid; } // use row i for(int i = 1; i < n-1; i++) { bool valid = true; for(int j = 0; j < n; j++) { if(g[i][j] == 'H') valid = false; if(j < i && g[j][0] == 'H') valid = false; if(j > i && g[j][n-1] == 'H') valid = false; } ret += valid; } } if(k >= 3) { for(int i = 1; i < n-1; i++) { for(int j = 1; j < n-1; j++) { // RDRD bool valid = g[i][j] == '.'; for(int a = 0; a < n; a++) { if(a <= i && g[a][j] == 'H') valid = false; if(a >= i && g[a][n-1] == 'H') valid = false; if(a <= j && g[0][a] == 'H') valid = false; if(a >= j && g[i][a] == 'H') valid = false; } ret += valid; valid = g[i][j] == '.'; // DRDR for(int a = 0; a < n; a++) { if(a <= i && g[a][0] == 'H') valid = false; if(a >= i && g[a][j] == 'H') valid = false; if(a <= j && g[i][a] == 'H') valid = false; if(a >= j && g[n-1][a] == 'H') valid = false; } ret += valid; } } } cout << ret << "\n"; } int main() { int t; cin >> t; while(t--) solve(); }

We can also solve this problem in $\mathcal{O}(N^2K)$ time by storing for each square, each possible number of turns (up to $K$), and each of the directions D and R, the number of ways for Bessie to reach that square using exactly that number of turns such that the last direction in which she walked was that direction. However, this is outside of the scope of both Bronze and Silver.