思想：维护一个区间

JAVA

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

public class Main {
    static List<Seg> segs = new ArrayList<Seg>();
    static class Seg{
        int l, r;
        public Seg(int l, int r) {
            this.l = l; this.r = r;
        }
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        for (int i = 0; i < n; i++) {
            segs.add(new Seg(in.nextInt(), in.nextInt()));
        }
        segs.sort((a, b) -> Integer.compare(a.l, b.l));  // 按区间左端升序

        int L = segs.get(0).l, R = segs.get(0).r;
        int res = 1;
        for (Seg seg : segs) {
            if (seg.l <= R) R = Math.max(R, seg.r);
            else {
                res ++;
                L = seg.l; R = seg.r;
            }
        }

        System.out.println(res);
    }

}

C++

#include <iostream>
#include <algorithm>

using namespace std;

typedef pair<int, int> PII;

const int N = 100010;

int n;
PII segs[N];

int main()
{
    cin >> n;
    for (int i = 0; i < n; i ++ ) cin >> segs[i].first >> segs[i].second;

    sort(segs, segs + n);

    int start = segs[0].first, end = segs[0].second;  // 维护 [start, end]
    int res = 1;
    for (auto seg : segs)
    {
        int l = seg.first, r = seg.second;
        if (r <= end) continue;  // 覆盖 不需要更新
        if (l <= end && r > end) end = r;  // 有交集 修改end
        else  // 无交集 更新
        {
            res ++;
            start = l, end = r;
        }
    }

    cout << res << endl;

    return 0;
}