Prim O(N^2)

const
  s = 5001;

var
  g:array[0..s,0..s] of longint;
  used:array[0..s] of boolean;
  d:array[0..s] of longint;
  i,j,n,m,u,v,x,y,z,min,mst:longint;

begin
  read(n,m);
  for i:=1 to m do
    begin
      read(x,y,z);
      g[x][y]:=z;
      g[y][x]:=z;
    end;
  for i:=2 to n do
    d[i]:=maxlongint;
  for i:=1 to n do
    begin
      min:=maxlongint;
      for j:=1 to n do
        if (d[j] < min) and (not used[j]) then
          begin
            min:=d[j];
            u:=j;
          end;
      used[u]:=true;
      inc(mst,d[u]);
      for j:=1 to n do
        if (d[j] > g[u][j]) and (not used[j]) then
          d[j]:=g[u][j]
    end;
  write(mst);
end.