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 Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)


import java.io.*;
import java.util.*;
import static java.lang.Math.*;
import static java.util.Arrays.*;

/**
 * @author AnonymousP
 * @__WHEN YOU FEEL LIKE QUITTING, THINK ABOUT WHY YOU STARTED__@
 */
//COMBINATON = nCr = n*(n-1)/2
public class Main {

    public static void main(String[] args) throws IOException {

        int n = sc.nextInt();
        int max = sc.nextInt();
        int start = max;
        int c = 0;
        for (int i = 0; i < n - 1; i++) {
            int next = sc.nextInt();
            if (next < start) {
                c += start - next;
            } else {
                start = next;
            }
        }

        prln(c);

        close();
    }

    static FastReader sc = new FastReader();
    static PrintWriter __out = new PrintWriter(new OutputStreamWriter(System.out));
    static StringTokenizer input;
    static Random __rand = new Random();

    // input
    //*******FAST IO*************FAST IO***************FAST IO****************//
    static class FastReader {

        BufferedReader br;
        StringTokenizer st;

        public FastReader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
// output

    static void pr(int i) {
        __out.print(i);
    }

    static void prln(int i) {
        __out.println(i);
    }

    static void pr(long l) {
        __out.print(l);
    }

    static void prln(long l) {
        __out.println(l);
    }

    static void pr(double d) {
        __out.print(d);
    }

    static void prln(double d) {
        __out.println(d);
    }

    static void pr(char c) {
        __out.print(c);
    }

    static void prln(char c) {
        __out.println(c);
    }

    static void pr(char[] s) {
        __out.print(new String(s));
    }

    static void prln(char[] s) {
        __out.println(new String(s));
    }

    static void pr(String s) {
        __out.print(s);
    }

    static void prln(String s) {
        __out.println(s);
    }

    static void pr(Object o) {
        __out.print(o);
    }

    static void prln(Object o) {
        __out.println(o);
    }

    static void prln() {
        __out.println();
    }

    static void pryes() {
        __out.println("yes");
    }

    static void pry() {
        __out.println("Yes");
    }

    static void prY() {
        __out.println("YES");
    }

    static void prno() {
        __out.println("no");
    }

    static void prn() {
        __out.println("No");
    }

    static void prN() {
        __out.println("NO");
    }

    static void pryesno(boolean b) {
        __out.println(b ? "yes" : "no");
    }

    ;
    static void pryn(boolean b) {
        __out.println(b ? "Yes" : "No");
    }

    static void prYN(boolean b) {
        __out.println(b ? "YES" : "NO");
    }

    static void prln(int... a) {
        for (int i = 0, len = a.length - 1; i < len; __out.print(a[i]), __out.print(' '), ++i);
        __out.println(a[a.length - 1]);
    }

    static void prln(long... a) {
        for (int i = 0, len = a.length - 1; i < len; __out.print(a[i]), __out.print(' '), ++i);
        __out.println(a[a.length - 1]);
    }

    static <T> void prln(Collection<T> c) {
        int n = c.size() - 1;
        Iterator<T> iter = c.iterator();
        for (int i = 0; i < n; __out.print(iter.next()), __out.print(' '), ++i);
        if (n >= 0) {
            __out.println(iter.next());
        } else {
            __out.println();
        }
    }

    static void h() {
        __out.println("hlfd");
        flush();
    }

    static void flush() {
        __out.flush();
    }

    static void close() {
        __out.close();
    }
}