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;
}