Labyrinth cses code example

Example: Labyrinth cses

#include <bits/stdc++.h>
using namespace std;
#define ii pair<int, int>#define f first#define s second#define mp make_pair
int n, m;char A[1000][1000];bool vis[1000][1000];
// previousStep stores the previous direction that we moved in to arrive that this cellint previousStep[1000][1000];
// 0 = up, 1 = right, 2 = down, 3 = leftint dx[4] = { -1, 0, 1, 0 };int dy[4] = { 0, 1, 0, -1 };string stepDir = "URDL";
int main() {    cin >> n >> m;
    queue<ii> q;    ii begin, end;    for (int i = 0; i < n; i++) {        for (int j = 0; j < m; j++) {            cin >> A[i][j];            if (A[i][j] == 'A') {                begin = mp(i, j);            } else if (A[i][j] == 'B') {                end = mp(i, j);            }        }    }
    q.push(begin);    vis[begin.f][begin.s] = true;
    while (!q.empty()) {        ii u = q.front(); q.pop();        for (int i = 0; i < 4; i++) {            ii v = mp(u.f + dx[i], u.s + dy[i]);            if (v.f < 0 || v.f >= n || v.s < 0 || v.s >= m) continue;            if (A[v.f][v.s] == '#') continue;            if (vis[v.f][v.s]) continue;            vis[v.f][v.s] = true;            previousStep[v.f][v.s] = i;            q.push(v);        }    }
    if (vis[end.f][end.s]) {        cout << "YES" << endl;        vector<int> steps;        while (end != begin) {            int p = previousStep[end.f][end.s];            steps.push_back(p);            // undo the previous step to get back to the previous square            // Notice how we subtract dx/dy, whereas we added dx/dy before            end = mp(end.f - dx[p], end.s - dy[p]);        }        reverse(steps.begin(), steps.end());
        cout << steps.size() << endl;        for (char c : steps) {            cout << stepDir[c];        }        cout << endl;    } else {        cout << "NO" << endl;    }
    return 0;}