## Apple Interview Question for Backend Developers

• 3

Country: United States
Interview Type: Phone Interview

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3
of 3 vote

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Solution
For the first problem a hash set of {IPs} may simply solve it.

However in the follow-ups as the amount of data cumulates, if the IPs are stored as strings, 4GB is becomes absolutely insufficient, since there are over 4 billion IPs out there and each ip as a string takes at least 9 bytes.
Even if convert ip to integer(8B), it wouldn't be enough.

An approach is to go with bit set. Then each ip takes only 1 bit. Have a bit set to store whether an ip occurs and another set to store whether an ip repeats.

This takes 2 * 2^32 bit = ~1GB -> fits in RAM.

``````public class IPFilter {

long[] map;          //mark all ip that showed up
long[] repeatedIP;    //mark all ips that repeatedly showed up

public IPFilter() {
//there's 2^32 IP in total.
// each long integer is identifies 64 IPs.
// Need 2^32 / 2^6 long integers in the bit map
int size = 1 << (32 - 6);
map = new long[size];
repeatedIP = new long[size];
}

for(String ip: IPs) {
long decimal = IPToLong(ip);
int idx = (int)(decimal / 64);
int res = (int)(decimal % 64);

if((map[idx] >> res) == 1) {    //repeated ip
repeatedIP[idx] |= (1 << res);
} else {                        //first occurred ip
map[idx] |= (1 << res);
}
}
}

private long IPToLong(String ipAddress) { //convert ip (base 256) to decimal
long result = 0;
for (int i = 0; i < ipAddressInArray.length; i++) {
int power = 3 - i;
result += ip * Math.pow(256, power);
}
return result;
}
}``````

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0
of 0 vote

@aonecoding, I believe

``if((map[idx] >> res) == 1``

should be

``if((map[idx] >> res) % 10 == 1``

. This will get the bit value at location res from the bitset

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-1
of 1 vote

It is better to avoid writing a single line of code, when you can write it in 2 lines.
[ stackoverflow.com/questions/618378/select-unique-or-distinct-values-from-a-list-in-unix-shell-script ]

``./yourscript_to_isolate_ip.sh | sort | uniq -u``

and this, will run no matter how less memory you have.
Aws guys actually run it, so does azure folks. I am pretty sure Apple will do the same.

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