Question

Suppose we have an array in the size of n, with values range from 0 to n-1.
A function that validates that there are no duplications of values is needed, without using another array.

Prototype: bool UniqueValues(unsigned arr[], size_t n);

This is an algorithm question for job interviews, please don’t suggest using built-in std functions.

Answer 1

Traverse the array and for each element, swap arr[i] with arr in the index of arr[i]( e.g, if arr[1]=2, swap it with arr[2], so arr[2]=2).
The indication whether the value is duplicated is that arr[ arr[i] ] == arr[i]. (found 2 somewhere when there is already 2 at arr[2]).

bool UniqueValues(unsigned arr[], size_t n)
{
    int val;
    for(size_t i=0;i<n;i++)
    {
        val = arr[i];
        if(val == arr[val] )  { if( val != i) return false; }  //  If the element is duplicated
        else {
            swap( arr[i], arr[val] );  //  Move the element to the index matching its value
            val = arr[i];
            if(val == arr[val] )  return false;  //  Another check for the swapped element
        }
    }
    return true;
}