uva 10047 Problem A: The Monocycle (搜索:BFS+优先队列)

一道很简单的BFS题目...居然用了两个小时


原因就是开始题目没有读清楚,原来转向还需要1S

而且也没有用优先队列,所以我的结果不是最优的

最后想通了就好做了

好像我的做法和很多人都不一样,别人的思路都是先让车子转向后进栈

在出栈的时候再执行前进操作

而我是直接分四个方向判断,感觉这样虽然代码长了些,但是思路很清晰,很容易理解

代码如下:

#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define MAXN 30
#define LL long long
using namespace std;

/* col含义:  |   dir含义:
* 0 - 绿色  |   0 - 北
* 1 - 黑色  |   1 - 西
* 2 - 红色  |   2 - 南
* 3 - 蓝色  |   3 - 东
* 4 - 白色  |
*/

int m, n, ans;
int g[MAXN][MAXN];
bool vis[MAXN][MAXN][MAXN][MAXN];

struct Position {
int x, y, dir, col, time;
bool operator  < (const Position& rhs) const {
return time > rhs.time;
}
}start, end;

bool BFS(Position start, Position end) {

Position tmp, cur;
priority_queue<Position> q;

memset(vis, 0, sizeof(vis));

start.col = 0;
start.dir = 0;
start.time = 0;
end.col = 0;
q.push(start);

while(!q.empty()) {
tmp = q.top();
q.pop();
vis[tmp.x][tmp.y][tmp.dir][tmp.col] = true;
//        cout << "tmp.x = " << tmp.x << "\ttmp.y = " << tmp.y << endl << endl;

if(tmp.x==end.x && tmp.y==end.y && tmp.col==end.col) {
ans = tmp.time;
return true;
}

if(tmp.x-1>=0 && g[tmp.x-1][tmp.y]==0 && !vis[tmp.x-1][tmp.y][0][(tmp.col+1)%5]) {//车子向北走
cur.x = tmp.x-1;
cur.y = tmp.y;
cur.col = (tmp.col+1)%5;
cur.dir = 0;
cur.time = tmp.time+1;

if(tmp.dir == 2)
cur.time += 2;

else if(tmp.dir != 0)
cur.time++;

q.push(cur);
}

if(tmp.y-1>=0 && g[tmp.x][tmp.y-1]==0 && !vis[tmp.x][tmp.y-1][1][(tmp.col+1)%5]) {//车子向西走
cur.x = tmp.x;
cur.y = tmp.y-1;
cur.col = (tmp.col+1)%5;
cur.dir = 1;
cur.time = tmp.time+1;

if(tmp.dir == 3)
cur.time += 2;
else if(tmp.dir != 1)
cur.time++;

q.push(cur);
}

if(tmp.x+1<m && g[tmp.x+1][tmp.y]==0 && !vis[tmp.x+1][tmp.y][2][(tmp.col+1)%5]) {//车子向南走
cur.x = tmp.x+1;
cur.y = tmp.y;
cur.col = (tmp.col+1)%5;
cur.dir = 2;
cur.time = tmp.time+1;

if(tmp.dir == 0)
cur.time += 2;
else if(tmp.dir != 2)
cur.time++;

q.push(cur);
}

if(tmp.y+1<n && g[tmp.x][tmp.y+1]==0  && !vis[tmp.x][tmp.y+1][3][(tmp.col+1)%5]) {//车子向东走
cur.x = tmp.x;
cur.y = tmp.y+1;
cur.col = (tmp.col+1)%5;
cur.dir = 3;
cur.time = tmp.time+1;

if(tmp.dir == 1)
cur.time += 2;
else if(tmp.dir != 3)
cur.time++;

q.push(cur);
}
}
return false;
}

int main(void) {
char ch;
int cnt = 1;
while(scanf("%d%d", &m, &n) && m+n) {

memset(g, 0, sizeof(g));

for(int i=0; i<m; ++i) {
getchar();
for(int j=0; j<n; ++j) {
ch = getchar();
if(ch == 'S') {
start.x = i;
start.y = j;
g[i][j] = 0;
} else if(ch == 'T') {
end.x = i;
end.y = j;
g[i][j] = 0;
} else if(ch == '#') {
g[i][j] = -1;
} else {
g[i][j] = 0;
}
}
}
if(cnt != 1)
cout << endl;
printf("Case #%d\n", cnt++);
if(BFS(start, end)) {
printf("minimum time = %d sec\n", ans);
} else cout << "destination not reachable" << endl;
}
return 0;
}