Monthly Archives: December 2014

Unique number

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Vinay Singh

Computer Teacher at Saint John's Academy
I am a GNU/Linux enthusiast and a computer science teacher by profession based at Allahabad, India. My hobbies include reading (mostly computer related stuff) and photography.
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A unique number can be defined as a positive integer without leading zeros, in which each digit occurs only once (no duplicate digit). There can be various methods to deunique numbertect whether or not a given positive integer is a unique number or not. I won’t be discussing methods which requires the number to be converted into a string as that would be inefficient in terms of space and time.

The approach which I am going to demonstrate in this post would use a boolean array of size ten to keep track for each of the possible ten digits. The idea is to extract digits of the number one by one, check the digit against the array to ascertain whether or not the extracted digit has already been encountered before and hence determine if the number is a unique number or not.

The java program to implement the above mentioned logic for unique number is as follows:

class UniqueNumber{
    public static boolean isUniqueNumber( int n ){
        boolean unique[] = new boolean[ 10 ];
        int digit;
        for( int temp=n; temp>0; temp/=10 ){
            if( unique[digit] ) return false;
            else unique[digit]=true;
        return true;
    public static void main( String args[] ){
        System.out.println(isUniqueNumber(1234)); //1234 is a unique number
        System.out.println(isUniqueNumber(12342));//12342 is not a unique number

The output of the above program to ascertain the unique number would be as follows:


The boolean array unique is of size ten so as to store the status of occurrence of any of the ten possible digits. The function isUniqueNumber() would return false as soon as a repeated digit is encountered.