Algorithm: efficient way to remove duplicate integers from an array

Question

I got this problem from an interview with Microsoft.

Given an array of random integers, write an algorithm in C that removes duplicated numbers an

Answer 1

This is the naive (N*(N-1)/2) solution. It uses constant additional space and maintains the original order. It is similar to the solution by @Byju, but uses no if(){} blocks. It also avoids copying an element onto itself.

#include 
#include 

int numbers[] = {4, 8, 4, 1, 1, 2, 9};
#define COUNT (sizeof numbers / sizeof numbers[0])

size_t undup_it(int array[], size_t len)
{
size_t src,dst;

  /* an array of size=1 cannot contain duplicate values */
if (len <2) return len; 
  /* an array of size>1 will cannot at least one unique value */
for (src=dst=1; src < len; src++) {
        size_t cur;
        for (cur=0; cur < dst; cur++ ) {
                if (array[cur] == array[src]) break;
                }
        if (cur != dst) continue; /* found a duplicate */

                /* array[src] must be new: add it to the list of non-duplicates */
        if (dst < src) array[dst] = array[src]; /* avoid copy-to-self */
        dst++;
        }
return dst; /* number of valid alements in new array */
}

void print_it(int array[], size_t len)
{
size_t idx;

for (idx=0; idx < len; idx++)  {
        printf("%c %d", (idx) ? ',' :'{' , array[idx] );
        }
printf("}\n" );
}

int main(void) {    
    size_t cnt = COUNT;

    printf("Before undup:" );    
    print_it(numbers, cnt);    

    cnt = undup_it(numbers,cnt);

    printf("After undup:" );    
    print_it(numbers, cnt);

    return 0;
}