POJ 1716 (差分约束）

Program P1716;
var
n,i,j,minq,maxq:longint;
d:array[-1..10000] of longint;
w:array[1..30000,1..2] of longint;
flag:boolean;
function min(a,b:longint):longint;
begin
if a<b then exit(a) else exit(b);
end;
function max(a,b:longint):longint;
begin
if a>b then exit(a) else exit(b);
end;

procedure relax(v,u,w:longint);
begin
if (d[u]+w<d[v]) then
begin
d[v]:=d[u]+w;
flag:=false;
end;
end;
procedure bellman_ford;
var
i,j,k:longint;
begin
//d[w[i,2]] - d[w[i,1]-1]>=2 ->d[w[i,1]-1]-d[w[i,2]]<=-2
//d[i]-d[i-1]<=1
//d[i]-d[i-1]>=0 ->d[i-1]-d[i]<=0

d[minq-1]:=0;
while (true) do
begin
flag:=true;
//    for i:=minq to maxq do relax(i,minq-1,0);
for i:=1 to n do relax(w[i,1]-1,w[i,2],-2);
for i:=minq to maxq do relax(i,i-1,1);
for i:=maxq downto minq do relax(i-1,i,0);
if flag then break;
end;
end;
begin
while not seekeof do
begin
minq:=maxlongint;  maxq:=0;
fillchar(d,sizeof(d),0);
read(n);
for i:=1 to n do
begin
read(w[i,1],w[i,2]);
minq:=min(minq,w[i,1]);
maxq:=max(maxq,w[i,2]);
end;
bellman_ford;
writeln(d[maxq]-d[minq-1]);
end;
end.