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 (Maybe slightly different)

import java.util.HashMap;

public class ArrayPairSum {


public static void main(String[] args) {        

    int []a = {2,45,7,3,5,1,8,9};
    printSumPairs(a,10);        

}


public static void printSumPairs(int []input, int k){
    Map pairs = new HashMap();

    for(int i=0;i

For input = {2,45,7,3,5,1,8,9} and if Sum is 10

Output pairs:

3,7 
8,2
9,1

Some notes about the solution :

• We iterate only once through the array --> O(n) time
• Insertion and lookup time in Hash is O(1).
• Overall time is O(n), although it uses extra space in terms of hash.