Find missing element from one array comparing from other in single for loop

Question

Is it possible to find out one missing element from one array that is not present in other array using single for loop in Java

e.g. a1 = {2,5,1,9,3,4} , length n+1

     a2 = {2,4,1,5,3}   , length n

missing element - 9 Using only a single for loop not by collections. Is it possible??

Sounds like an interview question. A few clarifications: Do you know the max/min values in the array beforehand? Are you only truly allowed only one for loop or the result just has to be O(n)? — Mshnik
– Mshnik, Commented Aug 10, 2014 at 7:55
yes its a interview question, max/min values are not given beforehand. The interviewer told me to solve using only one for loop, not even allowed to sort the array :( — siso
– siso, Commented Aug 10, 2014 at 8:02
If you KNOW that there is just one element in a1 that not in a2 then you can solve with simple math (subtract and delete) — odedsh
– odedsh, Commented Aug 10, 2014 at 8:03

Mshnik · Accepted Answer · 2014-08-10 19:32:51Z

9

At the most basic, if we are always talking about numbers, just sum the suckers.

int sumOne = 0;
int sumTwo = 0;

for(int i = 0; i < a1.length; i++){
    sumOne += a1[i];
    if(i < a2.length)
        sumTwo += a2[i];
}

int missingNumber = sumOne - sumTwo;

If the elements aren't always numbers... Ask for more loops.

edited Aug 10, 2014 at 19:32

answered Aug 10, 2014 at 8:05

Mshnik

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3 Comments

christopher Over a year ago

Just as an aside, it's worth noting that this will ONLY work for one missing number. If there is more than one missing number it will fail.

Mshnik Over a year ago

That's true. If you want more than that, you apparently can do it with some insane math: stackoverflow.com/questions/3492302/…

Abhishek Bansal Over a year ago

Why only integers? This solution can find any rational number IMO.

sujithvm · Accepted Answer · 2014-08-10 08:16:07Z

4

Find missing element using XOR

class Main {
    public static void main(String[] args) {
        int[] a1 = { 2, 5, 1, 9, 3, 4 };
        int[] a2 = { 2, 4, 1, 5, 3 };

        int missingElement = a1[a1.length - 1];
        for (int i = 0; i < a2.length; i++) {
            missingElement = missingElement ^ a1[i] ^ a2[i];
        }

        System.out.println(missingElement);
    }
}

edited Aug 10, 2014 at 8:16

answered Aug 10, 2014 at 8:10

sujithvm

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1 Comment

Apriori Over a year ago

Though overflow will correct itself via modular arithmetic with the summing solution, I find this solution to be more elegant.

user1935024 · Accepted Answer · 2014-08-10 08:06:48Z

2

sum the 2 arrays in one loop then subtract (max - min) = the missing element

answered Aug 10, 2014 at 8:06

user1935024

3 Comments

christopher Over a year ago

This has already been answered, better. Please either think of a different solution or remove your answer.

user1935024 Over a year ago

when i was writing it, some one else wrote !

christopher Over a year ago

That's fair enough! But we don't really like duplicate answers here. I guess now someone has upvoted it, there's no chance of you deleting it. Screw it. Have a +1.

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