Find a pair of elements from an array whose sum equals a given number

Question

Given array of n integers and given a number X, find all the unique pairs of elements (a,b), whose summation is equal to X.

The following is my solution, it is O(nLo

Answer 1

Implementation in Java : Using codaddict's algorithm:

import java.util.Hashtable;
public class Range {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    Hashtable mapping = new Hashtable();
    int a[]= {80,79,82,81,84,83,85};
    int k = 160;

    for (int i=0; i < a.length; i++){
        mapping.put(a[i], i);
    }

    for (int i=0; i < a.length; i++){
        if (mapping.containsKey(k - a[i]) && (Integer)mapping.get(k-a[i]) != i){
            System.out.println(k-a[i]+", "+ a[i]);
        }
    }      

}

}

Output:

81, 79
79, 81

If you want duplicate pairs (eg: 80,80) also then just remove && (Integer)mapping.get(k-a[i]) != i from the if condition and you are good to go.