Codeforces 158B Taxi Solution & Explanation

int n = int.Parse(Console.ReadLine());  //number of groups 

int[] arr = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
int carCnt = 0;

for(int i=0; i<arr.Length;i++){
    int num = arr[i];   // number of children in the group
    
    if(num==4){
        carCnt+=1;
        arr[i] = 0;
    }
    else if(num==3){
        var idx = Array.IndexOf(arr,1);
        if(idx>-1){
            carCnt+=1;
            arr[i] = 0;
            arr[idx] = 0;
        }
    }
    else if(num==2){
        var idx = Array.LastIndexOf(arr,2);
        if(idx>-1 && idx != i){
            carCnt+=1;
            arr[i] = 0;
            arr[idx] = 0;
            continue;
        }
        idx = Array.IndexOf(arr,1);
        if(idx>-1){
            arr[i] = 0;
            arr[idx] = 3;
        }
    }
    else if(num==1) {
        var idx = Array.IndexOf(arr,3);
        if(idx>-1){
            carCnt+=1;
            arr[i] = 0;
            arr[idx] = 0;
            continue;
        }
        
        idx = Array.LastIndexOf(arr,2);
        if(idx>-1){
            arr[i] = 0;
            arr[idx] = 3;
            continue;
        }
        
        idx = Array.LastIndexOf(arr,1);
        if(idx>-1 && idx != i){
            arr[i] = 0;
            arr[idx] = 2;
            continue;
        }
    }
    
}

int nonZeroCount = arr.Count(x => x > 0);
Console.WriteLine(carCnt+nonZeroCount);

int n = int.Parse(Console.ReadLine());  //number of groups 
 
int[] arr = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
int carCnt = 0;
int cnt1 = 0;
int cnt2 = 0;
int cnt3 = 0;


foreach(var a in arr){
    if(a==1){
        cnt1+=1;
    }
    else if(a==2){
        cnt2+=1;
    }
    else if(a==3){
        cnt3+=1;
    }
    else{   //4
        carCnt+=1;
    }
}

while(cnt3>0 && cnt1>0){
    cnt3-=1;
    cnt1-=1;
    carCnt+=1;
}

while(cnt1>1){
    cnt1-=2;
    cnt2+=1;
}

while(cnt2>1){
    cnt2-=2;
    carCnt+=1;
}

// If there is exactly one cnt1 and one cnt2, they can be combined into one taxi
if(cnt1==1 && cnt2==1){
    Console.WriteLine(carCnt+1+cnt3);
}
else{
    Console.WriteLine(carCnt+cnt1+cnt2+cnt3);
}