một số bài tập sách chuyên tin mọi người giúp ak

type arr = array[1..10000] of longint;
     arr2 = array[0..1, 1..20000] of longint;
var x, d : arr;
    n : integer;

procedure qsort(var a : arr2; L, R : integer);
var i, j, mid : integer;
    midV, midI, t : longint;
begin
    mid := L + ((R - L) shr 1);
    midV := a[0, mid]; midI := a[1, mid];
    i := L; j := R;
    while (i < j) do
        begin
            while ((a[0, i] < midV) or ((a[0, i] = midV) and (a[1, i] < midI))) do
                inc(i);
            while ((midV < a[0, j]) or ((midV = a[0, j]) and (midI < a[1, j]))) do
                dec(j);
            if (i <= j) then
                t := a[0, i]; a[0, i] := a[0, j]; a[0, j] := t;
                t := a[1, i]; a[1, i] := a[1, j]; a[1, j] := t;
                inc(i); dec(j);
        end;
    if (L < j) then
        qsort(a, L, j);
    if (i < R) then
        qsort(a, i, R);
end;

function cover(x, d : arr; n : integer) : longint;
var i, count : integer;
    left, answer : longint;
    a : arr2;
begin
    for i := 1 to n do
        begin
            a[0, (i shl 1) - 1] := x[i];
            a[1, (i shl 1) - 1] := -1;
            a[0, i shl 1] := x[i] + d[i];
            a[1, i shl 1] := 1;
        end;
        
    n := n shl 1;
    qsort(a, 1, n);
    
    answer := 0;
    count := a[1, 1];
    left := a[0, 1];
    i := 2;
    while (i <= n) do
        begin
            count := count + a[1, i];
            if (count = 0) then
                begin
                    answer := answer + a[0, i] - left;
                    inc(i);
                    if (i <= n) then
                        begin
                            left := a[0, i];
                            count := a[1, i];
                        end;
                end;
            inc(i);
        end;
    exit(answer);
end;