#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

typedef pair<int, int> PII;

vector<PII> segs;
int n;

void merge(vector<PII> &segs)
{
    vector<PII> res;

    int st = -2e9, ed = -2e9;
    for (int i = 0; i < segs.size(); i++)   //遍历vector的方法有点不同
    {
        int l = segs[i].first;
        int r = segs[i].second;

        if (ed < l)
        {
            if (st != -2e9) res.push_back({l, r});  //这个地方和y总写的有点不同
            st = l, ed = r;
        }
        else
            ed = max(ed, r);
    }

    if (st != -2e9) res.push_back({st, ed});

    segs = res;
}

int main()
{
    cin >> n;

    while (n--)
    {
        int l, r;
        cin >> l >> r;

        segs.push_back({l, r});
    }

    sort(segs.begin(), segs.end());

    merge(segs);

    cout << segs.size() << endl;

    return 0;
}

用迭代器写一下

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

typedef pair<int, int> PII;

vector<PII> segs;
int n;

void merge(vector<PII> &segs)
{
    vector<PII> res;

    int st = -2e9, ed = -2e9;

    vector<PII>::iterator it;
    for (it = segs.begin(); it< segs.end(); it++)
    {
        int l = (*it).first;
        int r = (*it).second;

        if (ed < l)
        {
            if (st != -2e9) res.push_back({l, r});
            st = l, ed = r;
        }
        else
            ed = max(ed, r);
    }

    if (st != -2e9) res.push_back({st, ed});

    segs = res;
}

int main()
{
    cin >> n;

    while (n--)
    {
        int l, r;
        cin >> l >> r;

        segs.push_back({l, r});
    }

    sort(segs.begin(), segs.end());

    merge(segs);

    cout << segs.size() << endl;

    return 0;
}