先对所有区间按左端点大小排序，然后分三种情况讨论来操作

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

vector<pair<int, int>> a;

inline bool cmp(pair<int, int>& x, pair<int, int>& y) {
    return x.first > y.first ? false : true;
}

int main()
{
    int n;
    cin >> n;
    for(int i = 0; i < n; ++i) {
        int l, r;
        cin >> l >> r;
        a.push_back({l, r});
    }

    int res = 1;
    sort(a.begin(), a.end(), cmp);
    //for(int i = 0; i < n; ++i)
        //cout << a[i].first << endl;
    pair<int, int> cur = a[0];
    for(int i = 1; i < n; ++i) {
        if(cur.second >= a[i].second) continue;
        else if(a[i].first <= cur.second) cur.second = a[i].second;
        else {
            ++res;
            cur = a[i];
        }
    }

    cout << res;
    return 0;
}