使用最小堆排序任务集，贪心策略为每次加入和上一个活动时间不冲突的，结束时间最早的活动


import heapq
n = int(input())
tasks = []
for _ in range(n):
    a, b = map(int, input().split())
    heapq.heappush(tasks, (b, a))
end = 0
count = 0
while tasks:
    b, a = heapq.heappop(tasks)
    if a >= end:
        count += 1
        end = b
print(count)