Problem:
Score : 300 points

Problem Statement

N persons are standing in a row. The height of the i-th person from the front is A_i.

We want to have each person stand on a stool of some heights - at least zero - so that the following condition is satisfied for every person:

Condition: Nobody in front of the person is taller than the person. Here, the height of a person includes the stool.

Find the minimum total height of the stools needed to meet this goal.

Constraints

1 \leq N \leq 2\times 10^5

1 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print the minimum total height of the stools needed to meet the goal.

Sample Input 1

5
2 1 5 4 3

Sample Output 1

4

If the persons stand on stools of heights 0, 1, 0, 1, and 2, respectively, their heights will be 2, 2, 5, 5, and 5, satisfying the condition.

We cannot meet the goal with a smaller total height of the stools.

Sample Input 2

5
3 3 3 3 3

Sample Output 2

0

Giving a stool of height 0 to everyone will work.

Solution in C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;

public class MainClass
{
    public static void Main(string[] args)
    {
#if DEBUG
        var inputs = new[] { @"
5
2 1 5 4 3
        ", @"
5
3 3 3 3 3
        ", @"

        ", @"

        ", @"

        " };

        var stream = Console.OpenStandardOutput();
        foreach (var str in inputs)
        {
            var text = str.TrimStart();
            if (text.Length == 0) continue;

            //File.ReadAllText(text);
            Console.SetIn(new StringReader(text));
            //using var stream = new FileStream(@$"tester\out_{text}", FileMode.Create);
            Main2(args, stream);
            Console.WriteLine("----------------------------------");
        }
#else
        Main2(args, Console.OpenStandardOutput());
#endif
    }

    ///<summary>10^9 + 7</summary>
    private const long BIGNUM = 1000000007L;

    private static void Main2(string[] args, Stream stream)
    {
        var sw = new StreamWriter(stream, Console.Out.Encoding);
        sw.AutoFlush = false;
        var beforeOut = Console.Out;
        Console.SetOut(sw);

        checked
        {
            var N = int.Parse(Console.ReadLine());
            var A = Console.ReadLine().Split().Select(long.Parse).ToList();

            var res = 0L;
            for (int i = 1; i < N; i++)
            {
                if (A[i] < A[i - 1])
                {
                    res += A[i - 1] - A[i];
                    A[i] = A[i - 1];
                }
            }

            Console.WriteLine(res);
        }

        Console.Out.Flush();
        Console.SetOut(beforeOut);
    }

    #region "入力"

    private static string NextInput = "";

    private static int Int() => int.Parse(Str());
    private static long Long() => long.Parse(Str());
    private static float Float() => float.Parse(Str());
    private static double Double() => double.Parse(Str());
    private static string Str()
    {
        var s = NextInput;
        if (s == "") s = Console.ReadLine();

        var idx = s.IndexOf(' ');
        if (idx >= 0)
        {
            NextInput = s.Substring(idx + 1);
            return s.Substring(0, idx);
        }
        NextInput = "";
        return s;
    }

    #endregion

}

public static class ComparerPlus
{
    ///<summary>セレクタ関数から昇順のComparerを生成する。</summary>
    public static Comparer<T> By<T, U>(Func<T, U> selector) where U : IComparable
    {
        return Comparer<T>.Create((a, b) => selector(a).CompareTo(selector(b)));
    }

    ///<summary>セレクタ関数から降順のComparerを生成する。</summary>
    public static Comparer<T> ByDecending<T, U>(Func<T, U> selector) where U : IComparable
    {
        return Comparer<T>.Create((a, b) => selector(b).CompareTo(selector(a)));
    }
}

public static class EnumerablePlus
{
    ///<summary>出力文字列に変換。</summary>
    public static string MkString<T>(this IEnumerable<T> list, string sep = null)
    {
        if (sep == null) sep = " ";

        return list.Aggregate(
            new System.Text.StringBuilder(),
            (a, b) => ((a.Length > 0) ? a.Append(sep) : a).Append(b)
            ).ToString();
    }

    ///<summary>添字付きfor。</summary>
    public static void ForEachWithIdx<T>(this IEnumerable<T> list, Action<T, long> a)
    {
        long i = 0;
        foreach (var item in list)
        {
            a(item, i++);
        }
    }

    ///<summary>添字なしfor。</summary>
    public static void ForEach<T>(this IEnumerable<T> list, Action<T> a)
    {
        foreach (var item in list)
        {
            a(item);
        }
    }

    ///<summary>指定回数デリゲートを呼び出す。</summary>
    public static IEnumerable<T> Repeat<T>(long n, Func<T> f)
    {
        for (long i = 0; i < n; i++)
        {
            yield return f();
        }
    }

    ///<summary>条件付きMin。</summary>
    public static T MinBy<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
    {
        return xs.Aggregate((a, b) => key(a).CompareTo(key(b)) <= 0 ? a : b);
    }

    ///<summary>条件付きMax。</summary>
    public static T MaxBy<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
    {
        return xs.Aggregate((a, b) => key(a).CompareTo(key(b)) >= 0 ? a : b);
    }

    ///<summary>条件付きMax。複数の値を返す。</summary>
    public static List<T> MaxByList<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
    {
        return xs.Aggregate(
            new List<T>(),
            (list, b) =>
            {
                if (list.Count == 0) { list.Add(b); return list; }

                var aa = key(list[0]);
                var bb = key(b);

                if (aa.CompareTo(bb) < 0) list.Clear();

                if (aa.CompareTo(bb) <= 0) { list.Add(b); return list; }

                return list;
            });
    }

    ///<summary>複数の値を返す。</summary>
    public static List<T> MaxList<T>(this IEnumerable<T> xs) where T : IComparable<T>
    {
        return xs.MaxByList(o => o);
    }
}

public static class MathPlus
{
    ///<summary>n(>=0)をradix進数に変換する。桁の小さい順に返す。</summary>
    public static List<long> NRadix(long n, long radix)
    {
        if (n < 0) throw new ArgumentOutOfRangeException(nameof(n));
        if (radix < 2) throw new ArgumentOutOfRangeException(nameof(n));

        var res = new List<long>();
        if (n == 0L)
        {
            res.Add(0);
            return res;
        }

        while (n > 0L)
        {
            res.Add(n % radix);
            n /= radix;
        }

        return res;
    }

    ///<summary>べき乗。高速な実装である。</summary>
    public static long Power(long x, long n)
    {
        if (n == 0) return 1;
        if (n % 2 == 0) return Power(x * x, n / 2);
        return x * Power(x, n - 1);
    }

    ///<summary>約数。順番は不定。</summary>
    public static List<long> Divisor(long n)
    {
        var list = Prime(n).
            GroupBy(id => id).
            Select(pair => new { prime = pair.Key, power = pair.Count() }).
            ToList();

        var result = list.Aggregate(new List<long>(), (left, pair) =>
        {
            var list2 = Enumerable.
                Range(0, pair.power + 1).
                Aggregate(new List<long>(), (res, l) =>
                {
                    var next = res.Count > 0 ? res.Last() * pair.prime : 1L;
                    res.Add(next);
                    return res;
                });

            if (left.Count == 0) return list2.ToList();

            return list2.
                SelectMany(l => left.Select(m => l * m)).
                ToList();
        });

        return result;
    }

    ///<summary>順列の数。</summary>
    public static long Permutation(long n, long r)
        => Factorial(n, r);

    ///<summary>組合せの数。</summary>
    public static long Combination(long n, long r)
    {
        if (r == 0) return 1;

        return Factorial(n, r) / Factorial(r);
    }

    ///<summary>階乗。カウントも指定可能。</summary>
    public static long Factorial(long a, long count = long.MaxValue)
    {
        long res = 1;

        for (long i = 0; a > 0 && i < count; i++)
        {
            checked { res *= a; }
            --a;
        }

        return res;
    }

    ///<summary>素因数分解。昇順。</summary>
    public static List<long> Prime(long a)
    {
        var res = new List<long>();

        var a2 = Math.Ceiling(Math.Sqrt(a));
        for (long b = 2; b <= a2; ++b)
        {
            while (a % b == 0)
            {
                res.Add(b);
                a = a / b;
                a2 = Math.Ceiling(Math.Sqrt(a));
            }
        }

        if (a > 1) res.Add(a);
        return res;
    }

    ///<summary>最小公倍数。</summary>
    public static long Lcm(IEnumerable<long> list)
    {
        return list.Aggregate((a, b) => Lcm(a, b));
    }

    ///<summary>最小公倍数。</summary>
    public static long Lcm(long a, long b)
    {
        return checked(a * b) / Gcd(a, b);
    }

    ///<summary>最大公約数。</summary>
    public static long Gcd(IEnumerable<long> list)
    {
        return list.Aggregate((a, b) => Gcd(a, b));
    }

    ///<summary>最大公約数。</summary>
    public static long Gcd(long a, long b)
    {
        if (a < b)
            Gcd(b, a);

        while (b != 0)
        {
            var rem = a % b;
            a = b;
            b = rem;
        }

        return a;
    }

    ///<summary>整数の切り上げ。</summary>
    public static long Ceiling(long a, long b) => (a + (b - 1)) / b;

    ///<summary>区間内の合計。beginも１から？end含む。</summary>
    public static ulong Sum(ulong begin, ulong end)
    {
        var up = Sum(end);
        var low = Sum(begin - 1);
        return up - low;
    }

    ///<summary>1からの合計。</summary>
    public static ulong Sum(ulong end) => checked((end + 1) * end / 2);
}

public static class Util
{
    ///<summary>リスト内の値を移動する。</summary>
    public static void Move<T>(this IList<T> list, int from, int to)
    {
        var item = list[from];

        for (var idx = from; idx > to; --idx)
        {
            list[idx] = list[idx - 1];
        }

        list[to] = item;
    }

    ///<summary>最後の値か、デフォルト値を取得。</summary>
    public static T LastOrValue<T>(this IList<T> list, T value)
    {
        return list.Count > 0 ? list.Last() : value;
    }

    ///<summary>辞書をMultiSetとして使うときのAdd。</summary>
    public static void AddMulti<T>(this IDictionary<T, int> xs, T key)
    {
        if (xs.ContainsKey(key))
            xs[key]++;
        else
            xs.Add(key, 1);
    }

    ///<summary>値を交換する。</summary>
    public static void Swap(ref int x, ref int y)
    {
        int temp = x;
        x = y;
        y = temp;
    }

    public static void Assert(bool cond)
    {
        if (!cond) throw new Exception();
    }

    /// <summary>
    /// データの範囲に対する二分探索を行う。
    /// </summary>
    /// <param name="min">最小値。</param>
    /// <param name="max">最大値。</param>
    /// <param name="Pred">テスト値と目標値を比較する。テスト値が目標値以下/未満ならtrue。それ以外はfalse。</param>
    /// <param name="CalcCenter"><paramref name="min">と<paramref name="max">の中間値を求める処理。</param>
    /// <typeparam name="T">二分探索するデータ型。</typeparam>
    /// <returns><paramref name="Pred">の戻り値が変わる境界の値を返す。</returns>
    public static (T, T) BinarySearch<T>(T min, T max, Predicate<T> Pred, Func<T, T, T> CalcCenter)
        where T : IComparable<T>, IEquatable<T>
    {
        while (min.CompareTo(max) <= 0)
        {
            var center = CalcCenter(min, max);
            if (center.Equals(min) || center.Equals(max)) break;

            if (Pred(center))
                min = center;
            else
                max = center;
        }

        return (min, max);
    }

    public static (int, int) BinarySearch(int min, int max, Predicate<int> Pred)
    {
        return BinarySearch<int>(min, max, Pred, (a, b) => (a + b) / 2);
    }
}