정렬 풀이
citat => 6 5 3 1 0
index => 0 1 2 3 4
if(citations[i]>=i+1) answer=i+1;
int solution(vector<int> citations) {
int answer = 0;
sort(citations.begin(),citations.end(),greater<int>());
for(int i=0;i<citations.size();i++){
if(citations[i]>=i+1) answer=i+1;
}
return answer;
}