CodeForces615ABulbs（水题）

Description
Vasya wants to turn on Christmas lights consisting of m bulbs. Initially, all bulbs are turned off. There are n buttons, each of them is connected to some set of bulbs. Vasya can press any of these buttons. When the button is pressed, it turns on all the bulbs it's connected to. Can Vasya light up all the bulbs?

If Vasya presses the button such that some bulbs connected to it are already turned on, they do not change their state, i.e. remain turned on.

Input
The first line of the input contains integers n and m (1 ≤ n, m ≤ 100) — the number of buttons and the number of bulbs respectively.

Each of the next n lines contains xi (0 ≤ xi ≤ m) — the number of bulbs that are turned on by the i-th button, and then xi numbers yij (1 ≤ yij ≤ m) — the numbers of these bulbs.

Output
If it's possible to turn on all m bulbs print "YES", otherwise print "NO".

Sample Input
Input
3 4
2 1 4
3 1 3 1
1 2
Output
YES
Input
3 3
1 1
1 2
1 1
Output

NO

代码：

#include<stdio.h>#include<iostream>#include<algorithm>#include<string.h>using namespace std;int main(){int n,m;while(scanf("%d %d",&n,&m)!=EOF){int a[1001];memset(a,0,sizeof(a));for(int i=1;i<=n;i++){int j;scanf("%d",&j);while(j--){int h;scanf("%d",&h);a[h]=1;}}int ans=0;for(int i=1;i<=m;i++){if(a[i]==1)ans++;}if(ans==m)printf("YES\n");elseprintf("NO\n");}return 0;}

思路：告诉你有一些按钮，每个按钮按了后，后面跟着的灯就会亮。你可以任意按按钮并且灯亮了就不会灭问你能不能把所有的灯点亮。

直接定义一个数组对每一盏灯保存为0，如果后续有按钮可以使这个灯亮，保存为1，最后判断全部的灯是否都为1，是的话输出YES，否则输出NO。