Problem:
Score : 300 points

Problem Statement

In a flower bed, there are N flowers, numbered 1,2,......,N. Initially, the heights of all flowers are 0.
You are given a sequence h=\{h_1,h_2,h_3,......\} as input. You would like to change the height of Flower k to h_k for all k (1 \leq k \leq N), by repeating the following "watering" operation:

Specify integers l and r. Increase the height of Flower x by 1 for all x such that l \leq x \leq r.

Find the minimum number of watering operations required to satisfy the condition.

Constraints

1 \leq N \leq 100

0 \leq h_i \leq 100

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
h_1 h_2 h_3 ...... h_N

Output

Print the minimum number of watering operations required to satisfy the condition.

Sample Input 1

4
1 2 2 1

Sample Output 1

2

The minimum number of watering operations required is 2.
One way to achieve it is:

Perform the operation with (l,r)=(1,3).

Perform the operation with (l,r)=(2,4).

Sample Input 2

5
3 1 2 3 1

Sample Output 2

5

Sample Input 3

8
4 23 75 0 23 96 50 100

Sample Output 3

221

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class Main {

    int count = 0;
    void solve() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        List<Integer> heights = new ArrayList<>();

        for (int i = 0; i < N; i++) {
            heights.add(sc.nextInt());
        }
        calcCount(heights);
        System.out.print(count);
    }

    void calcCount(List<Integer> heights) {
        // 脱出条件
        if (heights.size() == 0) {
            return;
        }
        if (heights.size() == 1) {
            count += heights.get(0);
            return;
        }

        int minHeight = Collections.min(heights);
        if (minHeight > 0) {
            // Integer 型のまま扱えば、もっと簡単にできるかも。
            for (int i = 0; i < heights.size(); i++) {
                int height = heights.get(i);
                heights.set(i, height - minHeight);
            }
            count += minHeight;
        }

        List<Integer> subHeights = new ArrayList<>();
        for (int i = 0; i < heights.size(); i++) {
            int height = heights.get(i);
            // 0 が来て一区切りなので、ここまでをcalcCount()にかける。
            if (height == 0) {
                if (subHeights.size() != 0) {
                    calcCount(subHeights);
                    subHeights.clear();
                }
            }
            // 部分集合の最後まで来たので、ここまでをcalcCount()にかける。
            else if (i == (heights.size() - 1)) {
                subHeights.add(height);
                calcCount(subHeights);
                subHeights.clear();
            }
            else {
                subHeights.add(height);
            }
        }
    }

    // Main
    public static void main(String[] args) {
        new Main().solve();
    }
}