Find repeating in O(n) and constant space [duplicate]

Answer 1

We can do it in O(n) and constant space:

1. For every element, calculate index = Math.abs(a[i]) - 1
2. Check the value at index
   • If it is positive, multiply by -1, i.e., make it negative.
   • if it is negative, return (index+1) as answer, as it means we have seen this index before.

""

static int findDup(int[] a){
    for(int i=0;i