Problem:
Score : 400 points

Problem Statement

There are N islands lining up from west to east, connected by N-1 bridges.

The i-th bridge connects the i-th island from the west and the (i+1)-th island from the west.

One day, disputes took place between some islands, and there were M requests from the inhabitants of the islands:

Request i: A dispute took place between the a_i-th island from the west and the b_i-th island from the west. Please make traveling between these islands with bridges impossible.

You decided to remove some bridges to meet all these M requests.

Find the minimum number of bridges that must be removed.

Constraints

All values in input are integers.

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq a_i < b_i \leq N

All pairs (a_i, b_i) are distinct.

Input

Input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M

Output

Print the minimum number of bridges that must be removed.

Sample Input 1

5 2
1 4
2 5

Sample Output 1

1

The requests can be met by removing the bridge connecting the second and third islands from the west.

Sample Input 2

9 5
1 8
2 7
3 5
4 6
7 9

Sample Output 2

2

Sample Input 3

5 10
1 2
1 3
1 4
1 5
2 3
2 4
2 5
3 4
3 5
4 5

Sample Output 3

4

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)



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

public class Main {

    static Scanner sc = new Scanner(System.in);


    public static void main(String[] args) {
        int n = sc.nextInt();
        int m = sc.nextInt();
        int[] c = new int[n + 1];
        List<Integer> aList = new ArrayList<>();
        List<Integer> bList = new ArrayList<>();
        int aMin = n;
        int bMax = 0;

        for (int i = 0; i < m; i++) {
            int a = sc.nextInt();
            int b = sc.nextInt();
            aList.add(a);
            bList.add(b);
            if (aMin > a) {
                aMin = a;
            }
            if (bMax < b) {
                bMax = b;
            }
            for (int j = a; j <= b; j++) {
                c[j]++;
            }
        }

        int answer = 0;
        while (aList.size() > 0) {
            answer++;
            //get max
            List<Integer> list = new ArrayList<>();
            int max = 0;
            int index = 0;
            for (int i = aMin; i <= bMax; i++) {
                if (max < c[i]) {
                    max = c[i];
                    index = i;
                }
                list.add(c[i]);
            }
            c[index] = 0;
            if (max == aList.size()) {
                break;
            }
            //済んだやつは消してく
            List<Integer> aList2 = new ArrayList<>();
            List<Integer> bList2 = new ArrayList<>();
            aMin = n;
            bMax = 0;
            for (int i = 0; i < aList.size(); i++) {
                int a = aList.get(i);
                int b = bList.get(i);
                if (a <= index && b > index) {
                    continue;
                }
                aList2.add(a);
                if (aMin > a) {
                    aMin = a;
                }
                bList2.add(b);
                if (bMax < b) {
                    bMax = b;
                }
            }
            aList = aList2;
            bList = bList2;
        }
        System.out.println(answer);
    }
}