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: Accepted)

import java.io.*;
import java.util.*;
public class Main {
 
  public static void main(String[] args) { 
        solve();
      out.close();
  }

  private static void solve() {
      int n = sc.nextInt();
      int[] arr = sc.readArray(n);
      long res = 0;
      int max = arr[0];
      for(int i = 1; i < n; i++) {
        if(arr[i] < max) {
          res += max - arr[i];
        } else {
          max = arr[i];
        }
      }
      out.println(res);
  }

  private static String mkArr(int[] arr) {
    StringBuilder sb = new StringBuilder();
    for(int a : arr) {
      sb.append(a + " ");
    }
    return sb.toString();
  }

  private static String mkList(List<Integer> arr) {
    StringBuilder sb = new StringBuilder();
    for(int a : arr) {
      sb.append(a + " ");
    }
    return sb.toString();
  }
  
  static int mod = 1_000_000_007;

    private static long C(int n, int k) {
    long res1 = factor(n-k+1, n);
    long res2 = factor(1, k);
    long reverse = reverse(res2);
    return (res1 * reverse) % mod;
  }

  private static long reverse(long a) {
    return powMod(a, mod - 2);
  }

  private static long factor(int left, int right){
    long res = 1;
    for(int i = left; i <= right; i++)
      res = (res * i) % mod;
    return res;
  }

  public static PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
 
  public static FastScanner sc = new FastScanner();

  static class FastScanner {
      BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
      StringTokenizer st=new StringTokenizer("");
      String next() {
        while (!st.hasMoreTokens())
          try {
            st=new StringTokenizer(br.readLine());
          } catch (IOException e) {
            e.printStackTrace();
          }
        return st.nextToken();
      }
      
      int nextInt() {
        return Integer.parseInt(next());
      }
      int[] readArray(int n) {
        int[] a=new int[n];
        for (int i=0; i<n; i++) a[i]=nextInt();
        return a;
      }
      long nextLong() {
        return Long.parseLong(next());
      }
  }

  static long pow(long a, long N) {
    if (N == 0) return 1;
    else if (N == 1) return a;
    else {
      long R = pow(a,N/2);
      if (N % 2 == 0) {
        return R*R;
      }
      else {
        return R*R*a;
      }
    }
  }
   
  static long powMod(long a, long N) {
    if (N == 0) return 1;
    else if (N == 1) return a % mod;
    else {
      long R = powMod(a,N/2) % mod;
      R = (R * R) % mod;
      if (N % 2 == 1) {
        R = (R * a) % mod;
      }
        return R % mod;
    }
  }

  static void mergeSort(int[] A){ // low to hi sort, single array only
    int n = A.length;
    if (n < 2) return;
    int[] l = new int[n/2];
    int[] r = new int[n - n/2];

    for (int i = 0; i < n/2; i++){
      l[i] = A[i];
    }

    for (int j = n/2; j < n; j++){
      r[j-n/2] = A[j];
    }

    mergeSort(l);
    mergeSort(r);
    merge(l, r, A);
  }
 
  static void merge(int[] l, int[] r, int[] a){
    int i = 0, j = 0, k = 0;
    while (i < l.length && j < r.length && k < a.length){
      if (l[i] < r[j]){
        a[k] = l[i];
        i++;
      }
      else{
        a[k] = r[j];
        j++;
      }
      k++;
    }
    while (i < l.length){
      a[k] = l[i];
      i++;
      k++;
    }

    while (j < r.length){
      a[k] = r[j];
      j++;
      k++;
    }
  }

  private static class Edge {
    int src;
    int dest;
    int weight;
    public Edge(int s, int d, int w) {
      this.src = s;
      this.dest = d;
      this.weight = w;
    }
  }
  private static int[] prim(Map<Integer, List<Edge>> graph) {
      int n = graph.size(); // number of the nodes
      Edge[] edges = new Edge[n];
      for(int i = 0; i < n; i++)
        edges[i] = new Edge(-1, i, Integer.MAX_VALUE);
      edges[0].weight = 0;

      int[] parents = new int[n];
      parents[0] = -1;

      TreeSet<Edge> treeSet = new TreeSet<>((a, b) -> a.weight - b.weight);
      treeSet.add(edges[0]);

      boolean[] visited = new boolean[n];
      while(treeSet.size() > 0) {
        Edge e = treeSet.pollFirst();
        visited[e.dest] = true;
        for(Edge child : graph.get(e.dest)) {
          if(visited[child.dest]) continue;
          if(edges[child.dest].weight > child.weight) {
            treeSet.remove(edges[child.dest]);
            edges[child.dest].weight = child.weight;
            treeSet.add(edges[child.dest]);
            parents[child.dest] = e.dest;
          }
        }
      }
      return parents;
  }

  private static void testDijkstr() {
    Map<Integer, List<Edge>> graph = new HashMap<>();
    for(int i = 0; i < 9; i++)
      graph.put(i, new ArrayList<>());
    addEdge(0, 1, 4 , graph);
    addEdge(0, 7, 8 , graph);
    addEdge(1, 2, 8 , graph);
    addEdge(1, 7, 11, graph);
    addEdge(2, 3, 7 , graph);
    addEdge(2, 8, 2 , graph);
    addEdge(2, 5, 4 , graph);
    addEdge(3, 4, 9 , graph);
    addEdge(3, 5, 14, graph);
    addEdge(4, 5, 10, graph);
    addEdge(5, 6, 2 , graph);
    addEdge(6, 7, 1 , graph);
    addEdge(6, 8, 6 , graph);
    addEdge(7, 8, 7 , graph);
    Edge[] es = dijkstra(graph);
    for(int i = 0; i < 9; i++)
    out.println(i + ": " + es[i].weight);
  }

  private static void addEdge(int a, int b, int w, Map<Integer, List<Edge>> graph) {
    graph.get(a).add(new Edge(a, b, w));
    graph.get(b).add(new Edge(b, a, w));
  }

  private static Edge[] dijkstra(Map<Integer, List<Edge>> graph) {
    int n = graph.size();
    Edge[] es =  new Edge[n];
    for(int i = 0; i < n; i++)
      es[i] = new Edge(-1, i, Integer.MAX_VALUE);
    es[0].weight = 0;

    TreeSet<Edge> treeSet = new TreeSet<>((a, b) -> a.weight - b.weight);
    treeSet.add(es[0]);

    boolean[] visited = new boolean[n];

    while(treeSet.size() > 0) {
      Edge e = treeSet.pollFirst();
      visited[e.dest] = true;
      for(Edge ed : graph.get(e.dest)) {
        if(visited[ed.dest] || ed.weight + es[e.dest].weight >= es[ed.dest].weight) 
          continue;
        treeSet.remove(es[ed.dest]);
        es[ed.dest].weight = ed.weight + es[e.dest].weight;
        treeSet.add(es[ed.dest]);
      }
    }
    return es;
  }

  private static Edge[] kruskal(int v, Edge[] edges) {
    Edge[] res = new Edge[v-1];

    int[] p = new int[v];
    for(int i = 0; i < v; i++) 
      p[i] = i;

    int[] rank = new int[v];
    Arrays.fill(rank, 1);

    Arrays.sort(edges, (e1, e2) -> e1.weight - e2.weight);
    for(int i = 0, idx = 0; i < res.length;) {
      Edge e = edges[idx++];
      if(union(p, rank, e.src, e.dest)) {
        res[i++] = e;
      }
    }
    return res;
  }

  private static boolean union(int[] root, int[] rank, int u, int v) {
      int rootA = find(root, u);
      int rootB = find(root, v);
      if(rootA == rootB) return false;

      if(rank[rootA] > rank[rootB]) {
        rank[rootA] += rank[rootB];
        root[rootB] = rootA;
      } else {
        rank[rootB] += rank[rootA];
        root[rootA] = rootB;
      }
      return true;
  }

  private static int find(int[] root, int node) {
    if(root[node] != node) {
      root[node] = find(root, root[node]);
    }
    return root[node];
  }

  static class BIT {
    int n;
    int[] nums;
    int[] bit;

    public BIT(int[] nums) {
      this.nums = nums;
      this.n = nums.length;
      this.bit = new int[n + 1];
      for(int i = 0; i < n; i++)
        init(i, nums[i]);
    }

    void init(int i, int val) {
      i++;
      while(i <= n) {
        bit[i] += val;
        i += (i & -i); // (i & (-i)): get moet least significate digit
      }
    }

    void update(int i, int val) {
      int diff = val - nums[i];
      nums[i] = val;
      init(i, diff);
    }

    int getSum(int i) {
      int sum = 0;
      i++;
      while(i > 0) {
        sum += bit[i];
        i -= (i & -i); // ex: 1100 -> 1000 -> 0000. subtract moet least significate digit
      }
      return sum;
    }

    int sumRange(int i, int j) {
      return getSum(j) - getSum(i - 1);
    }
  }
}