题解 | 校门外的树

L, M = list(map(int, input().split()))

arr = [1] * (L + 1)

for i in range(M):
    a, b = list(map(int, input().split()))
    for j in range(a, b + 1):
        arr[j] = 0

print(sum(arr))

哈希表解决这个问题