Bzoj 1295 scoi 2009 longest distance spfa

Here is a map with obstacles on some points. Now there is a t chance to remove the obstacle and ask how long is the longest Euclidean distance on the map.

Idea: create an image based on the source image. f [I] indicates the minimum number of obstacles to be removed from the start point. Then, use brute force enumeration to start the source point and update the answer.

Code:

#include <map>#include <queue>#include <cmath>#include <cstdio>#include <iomanip>#include <cstring>#include <iostream>#include <algorithm>#define MAX 10010using namespace std;const int dx[] = {0,1,-1,0,0};const int dy[] = {0,0,0,1,-1};map<int,pair<int,int> > G;int m,n,k;int head[MAX],total;int next[MAX],aim[MAX];bool is_block[MAX];int f[MAX];bool v[MAX];char src[110][110];int num[110][110],cnt;inline void Add(int x,int y){next[++total] = head[x];aim[total] = y;head[x] = total;}inline void SPFA(int start){static queue<int> q;while(!q.empty())q.pop();q.push(start);memset(v,false,sizeof(v));memset(f,0x3f,sizeof(f));f[start] = is_block[start];while(!q.empty()) {int x = q.front(); q.pop();v[x] = false;for(int i = head[x]; i; i = next[i])if(f[aim[i]] > f[x] + is_block[aim[i]]) {f[aim[i]] = f[x] + is_block[aim[i]];if(!v[aim[i]]) {v[aim[i]] = true;q.push(aim[i]);}}}}inline double Calc(pair<int,int> p1,pair<int,int> p2){return sqrt((double)(p1.first - p2.first) * (p1.first - p2.first) + (double)(p1.second - p2.second) * (p1.second - p2.second));}int main(){cin >> m >> n >> k;for(int i = 1; i <= m; ++i) {scanf("%s",src[i] + 1);for(int j = 1; j <= n; ++j) {num[i][j] = ++cnt;G[cnt] = make_pair(i,j);if(src[i][j] == '1')is_block[cnt] = true;}}for(int i = 1; i <= m; ++i)for(int j = 1; j <= n; ++j)for(int k = 1; k <= 4; ++k) {int fx = i + dx[k];int fy = j + dy[k];if(!num[fx][fy])continue;Add(num[i][j],num[fx][fy]);Add(num[fx][fy],num[i][j]);}double ans = .0;for(int i = 1; i <= cnt; ++i) {SPFA(i);for(int j = 1; j <= cnt; ++j)if(f[j] <= k)ans = max(ans,Calc(G[i],G[j]));}cout << fixed << setprecision(6) << ans << endl;return 0;}

Bzoj 1295 scoi 2009 longest distance spfa