次短路（非严格）

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这里的次短路是指最短路只有一条情况下的次短路，如果有两条及两条以上的最短路时则输出最短路题目

求次短路的思路就是首先求出最短路后，记录最短路的路径，枚举删变（注意数组模拟链表要删双向边）后再次SPFA，求出即可。

program pro;var        i,j,n,m,num,tot:longint;        min,cmin:real;        flag:boolean;        point:array[0..205]of record x,y:longint; end;        link:array[0..45000]of record ke,po,ne,br:longint; pr:real; bj:boolean; end;        pre:array[0..250]of record num,po:longint; end;        que:array[0..10000]of longint;        st:array[0..250]of longint;        dis:array[0..250]of real;        team:array[0..250]of boolean;function fd(x,y:longint):real;begin        exit(sqrt(sqr(point[x].x-point[y].x)+sqr(point[x].y-point[y].y)));end;procedure make(x,y:longint; z:real; br:longint);begin        inc(tot);        link[tot].ne:=st[x];        st[x]:=tot;        link[tot].po:=y;        link[tot].ke:=x;        link[tot].pr:=z;        link[tot].br:=br;end;procedure init;var        ii,u,v:longint;        c:real;begin        readln(n,m);        for ii:=1 to n do readln(point[ii].x,point[ii].y);        for ii:=1 to m do        begin                readln(u,v);                c:=fd(u,v);                make(u,v,c,1);                make(v,u,c,-1);        end;end;procedure spfa(x:longint);var        h,t,ke,po:longint;        temp:longint;begin        fillchar(que,sizeof(que),0);        fillchar(dis,sizeof(dis),$5f);        fillchar(team,sizeof(team),false);        dis[x]:=0; team[x]:=true; que[1]:=x; h:=0; t:=1;        while h<=t do        begin                inc(h);                ke:=que[h]; team[ke]:=false;                temp:=st[ke];                while temp<>0 do                begin                        po:=link[temp].po;                        if (not link[temp].bj)and(dis[po]>dis[ke]+link[temp].pr) then                        begin                                dis[po]:=dis[ke]+link[temp].pr;                                if not flag then                                begin                                        pre[po].po:=ke;                                        pre[po].num:=temp;                                end;                                if (not team[po]) then                                begin                                        inc(t);                                        que[t]:=po; team[po]:=true;                                end;                        end;                        temp:=link[temp].ne;                end;        end;        flag:=true;end;procedure main;var        po:longint;begin        flag:=false;        fillchar(pre,sizeof(pre),0);        spfa(1);        min:=dis[n]; cmin:=maxlongint;        po:=n;        while po<>1 do        begin              num:=pre[po].num;              link[num].bj:=true;              link[num+link[num].br].bj:=true;              spfa(1);              if dis[n]=min then              begin                        writeln(dis[n]:0:2);                        closef;                        halt;              end;              if (dis[n]>min)and(dis[n]<cmin) then cmin:=dis[n];              link[num].bj:=false;              link[num+link[num].br].bj:=false;              po:=pre[po].po;        end;        writeln(cmin:0:2);end;begin        init;        main;end.

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