题解：AT_abc405_d [ABC405D] Escape Route

#include <bits/stdc++.h>
#define int long long
//#define USE_FREOPEN
//#define MUL_TEST
#define FILENAME ""
using namespace std;
constexpr int N = 5005, M = 5005;
int dx[4] = {0, 1, 0, -1}; 
int dy[4] = {1, 0, -1, 0}; 
char da[4] = {'<', '^', '>', 'v'}; //各个方向对应箭头
char mp[N][M],ans[N][M];
bool vis[N][M];
queue<pair<int, int> > q;

void solve() {
	int n, m;
	cin >> n >> m;
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= m; j++) {
			cin >> mp[i][j];
			if (mp[i][j] == 'E') { //将每个紧急出口加入队列
				q.push(make_pair(i, j));
				vis[i][j] = 1;
			}
			ans[i][j] = mp[i][j];
		}
	}
	
	while (q.size()) { //BFS
		int x = q.front().first, y = q.front().second;
		q.pop();
		for (int i = 0; i < 4; i++) {
			int tx = x + dx[i], ty = y + dy[i];
			if (tx <= 0 || tx > n || ty <= 0 || ty > m || mp[tx][ty] == '#' || vis[tx][ty]) continue; //判边界、墙和已走过的位置
			q.push(make_pair(tx, ty));
			vis[tx][ty] = 1;
			ans[tx][ty] = da[i]; //写入答案
		}
	}

    //输出答案
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= m; j++)
			cout << ans[i][j];
		cout << '\n';
	}
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr); cout.tie(nullptr);

	#ifdef USE_FREOPEN
		freopen(FILENAME ".in","r",stdin);
		freopen(FILENAME ".out","w",stdout);
	#endif
	int _ = 1;
	#ifdef MUL_TEST
		cin >> _;
	#endif
	while (_--)
		solve();
	_^=_;
	return 0^_^0;
}