UVA 225 - Golygons（dfs回溯）

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题意：

一个探险队要去参观城市，走n次，每次走1,2,3,4,5,…n步，参观停留的城市，每次方向转90度。路途中有一些障碍物是不能走的，之前拐弯过的点也是不能走的。求所有走法，按字典序输出。

解析：

直接回溯深搜就可以了。

AC代码

#include <cstdio>#include <cstring>#include <string>#include <algorithm>using namespace std;const int N = 1000;const int LEN = 500;const char str[] = "nsew";//  N  S  W  Econst int dx[] = { 1,-1, 0, 0};const int dy[] = { 0, 0, 1,-1};string ans[N];int last, m, tot, path[N];bool grid[N][N], vis[N][N];bool can_move(int x, int y, int xx, int yy) {    int from, to;    if(x == xx) {        from = min(y, yy);        to = max(y, yy);        for(int i = from; i <= to; i++)            if(grid[xx][i]) return false;    }else if(y == yy) {        from = min(x, xx);        to = max(x, xx);        for(int i = from; i <= to; i++)            if(grid[i][yy]) return false;    }    return true;}void dfs(int x, int y, int dir, int cur) {    if(cur == last) {        if(x == LEN && y == LEN) {            ans[tot] = "";            for(int i = 0; i < last; i++)                ans[tot] += str[path[i]];            tot++;        }        return ;    }    int xx, yy, d;    if(dir == 0 || dir == 1) d = 2;    else d = 0;    for(int i = 0; i < 2; i++) {        xx = x + (cur+1)*dx[d+i];        yy = y + (cur+1)*dy[d+i];        if(!vis[xx][yy] && can_move(x, y, xx, yy)) {            path[cur] = d+i;            vis[xx][yy] = true;            dfs(xx, yy, d+i, cur+1);            vis[xx][yy] = false;        }    }}int main() {    //freopen("in.txt", "r", stdin);    int T;    scanf("%d", &T);    while(T--) {        memset(vis, 0, sizeof(vis));        memset(grid, 0, sizeof(grid));        scanf("%d%d",&last, &m);        int x, y;        for(int i = 0; i < m; i++) {            scanf("%d%d", &x, &y);            grid[y+LEN][x+LEN] = true;        }        tot = 0;        for(int i = 0; i < 4; i++) {            memset(vis, false, sizeof(vis));            path[0] = i;            dfs(LEN+dx[i], LEN+dy[i], i, 1);        }        sort(ans, ans+tot);        for(int i = 0; i < tot; i++) {            puts(ans[i].c_str());        }        printf("Found %d golygon(s).\n\n", tot);    }    return 0;}

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