Puzzle : finding out repeated element in an Array

Question

Size of an array is n.All elements in the array are distinct in the range of [0 , n-1] except two elements.Find out repeated element without using extra temporary array with con

Answer 1

Based on @sje's answer. Worst case is 2 passes through the array, no additional storage, non destructive.

O(n) without the temp array.

a[]={1,0,0,2,3};
i=0;
int required;
while (a[a[i] % n] < n)    
   a[a[i++] % n] += n;

required = a[i] % n;
while (i-->0)
   a[a[i]%n]-=n;

print required;

(Assuming of course that n < MAX_INT/2)