贪心策略：

每次选择结束时间最早的活动

代码： 

#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 10;
int n;
struct pp
{
    int a, b;
} p[N];
bool cmp(pp x, pp y)
{
    return x.b < y.b;
}
int ans = 0;

int main()
{
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> p[i].a >> p[i].b;
    }
    sort(p + 1, p + n + 1, cmp);
    int temp = 0;
    for (int i = 1; i <= n; i++)
    {
        if (p[i].a >= temp)
        {
           temp= p[i].b;
            ans++;
        }
    }
    cout << ans;
}