Codeforces 359E Neatness

Neatnes

dfs一下用set维护能不能走， 进入的时候点亮灯， 回溯的时候灭灯。

#include<bits/stdc++.h>
#define LL long long
#define LD long double
#define ull unsigned long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ALL(x) (x).begin(), (x).end()

using namespace std;

const int N = 500 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-8;
const double PI = acos(-1);

template<class T, class S> inline void add(T& a, S b) {a += b; if(a >= mod) a -= mod;}
template<class T, class S> inline void sub(T& a, S b) {a -= b; if(a < 0) a += mod;}
template<class T, class S> inline bool chkmax(T& a, S b) {return a < b ? a = b, true : false;}
template<class T, class S> inline bool chkmin(T& a, S b) {return a > b ? a = b, true : false;}

int n, sx, sy;
int on[N][N];
bool vis[N][N];
set<int> row[N], col[N];
string path;

bool check(int x, int y, int vx, int vy) {
    if(x + vx <= 0 || x + vx > n) return false;
    if(y + vy <= 0 || y + vy > n) return false;
    if(vis[x + vx][y + vy]) return false;
    if(!vy) {
        if(vx == 1) return upper_bound(ALL(col[y]), x) != col[y].end();
        else return lower_bound(ALL(col[y]), x) != col[y].begin();
    } else {
        if(vy == 1) return upper_bound(ALL(row[x]), y) != row[x].end();
        else return lower_bound(ALL(row[x]), y) != row[x].begin();
    }
}

void dfs(int x, int y) {
    vis[x][y] = true;
    if(!on[x][y]) {
        path.push_back('1');
        on[x][y] = true;
        row[x].insert(y);
        col[y].insert(x);
    }
    if(check(x, y, 0, 1)) {
        path.push_back('R');
        dfs(x, y + 1);
        path.push_back('L');
    }
    if(check(x, y, 0, -1)) {
        path.push_back('L');
        dfs(x, y - 1);
        path.push_back('R');
    }
    if(check(x, y, 1, 0)) {
        path.push_back('D');
        dfs(x + 1, y);
        path.push_back('U');
    }
    if(check(x, y, -1, 0)) {
        path.push_back('U');
        dfs(x - 1, y);
        path.push_back('D');
    }
    if(on[x][y]) {
        on[x][y] = false;
        path.push_back('2');
        row[x].erase(y);
        col[y].erase(x);
    }
}

int main() {
    cin >> n >> sx >> sy;
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= n; j++) {
            cin >> on[i][j];
            if(on[i][j]) {
                row[i].insert(j);
                col[j].insert(i);
            }
        }
    }
    dfs(sx, sy);
    for(int i = 1; i <= n; i++)
        for(int j = 1; j <= n; j++)
            if(on[i][j]) return puts("NO"), 0;
    cout << "YES" << "\n";
    cout << path << "\n";
    return 0;
}

/*
*/