Detect duplicate in a subset from a set of elements

Question

I have a set of numbers say : 1 1 2 8 5 6 6 7 8 8 4 2...

I want to detect the duplicate element in subsets(of given size say k) of the above numbers... For example : Consider the increasing subsets(for example consider k=3)

Subset 1 :{1,1,2}

Subset 2 :{1,2,8}

Subset 3 :{2,8,5}

Subset 4 :{8,5,6}

Subset 5 :{5,6,6}

Subset 6 :{6,6,7}

....

.... So my algorithm should detect that subset 1,5,6 contains duplicates..

My approach : 1)Copy the 1st k elements to a temporary array(vector) 2) using #include file in C++ STL...using unique() I would determine if there's any change in size of vector.. Any other clue how to approach this problem..

Answer 1

Take a subset, add each element into a hash table. If the hash table already contains a value - print out saying there is a duplicate.

A hash table will only allocate 1 memory block per entry. Hence its easy to find if the number already exists. Since these are just regular entries your table will be something like this:

struct Hashtable{
   int number;
};

static struct Hashtable hashTable[10];

int getHash(int x){ return x; }

void addHash(int number)
{
    if(hashTable[number].number == -1)
    { /*OK*/
        hashTable[number].number = number;
    }else{
        printf("DUPLICATE FOUND! BAILING OUT!");
    }
}

void initTable(){ for(int i = 0; i < 10; i++){hashTable[i].number = -1;}}

Answer 2

If they're all sets then by definition they don't contain duplicates. If they're eg. lists then you could just make sets of the sublists. In Python 2.x:

items = [1, 1, 2, 8, 5, 6, 6, 7, 8, 8, 4, 2]

for i in xrange(len(items) - 2):
    sublist = items[i:i + 3]
    if len(set(sublist)) < 3:
        print sublist, 'contains duplicates.'
    else:
        print sublist, 'contains no duplicates.'

Result:

[1, 1, 2] contains duplicates.
[1, 2, 8] contains no duplicates.
[2, 8, 5] contains no duplicates.
[8, 5, 6] contains no duplicates.
[5, 6, 6] contains duplicates.
[6, 6, 7] contains duplicates.
[6, 7, 8] contains no duplicates.
[7, 8, 8] contains duplicates.
[8, 8, 4] contains duplicates.
[8, 4, 2] contains no duplicates.