How can I find two elements in an array that sum to k

Question

Suppose you are given an array of unsorted integers as

A = {3,4,5,1,4,2}

Input : 6 Output : {5,1}, {4,2}

How can I do this in O(n) or O(log n). Any suggestions will be appreciated.

Update: Can we write something more efficient than this?

for(int i=0;i<array.length-1;i++)  
{  
    if(array[i]+array[i+1]==6)  
        System.out.println("{"+array[i]+","+array[i+1]+"}");  
}  

Answer 1

If the numbers stored in the input array are only positive then I'd create another array K of k+1 ArrayList elements. Where k is the number you need them to add up to. Only two numbers less than k can add up to k (assuming we deal with positive ints} or in special case {0,k}. Then I would iterate through all elements of input array and for each int m that is less or equal to k I'd take its index and add that index to the array of ArrayList K at index m. Then I would iterate through first half of the array K and for each index i that has some ints stored in it I would find complementary index [k-i] and see if there are any values in it. If there are then those are your pairs. And btw this is O(n).

public static void findElemtsThatSumTo( int data[], int k){
    List arrayK[]= new List[k+1];
    for(int i=0; i<arrayK.length; i++)
        arrayK[i]= new ArrayList<Integer>();

    for(int i=0; i<data.length; i++){
        if(data[i]<=k)
            arrayK[data[i]].add(i);
    }

    for(int i=0; i<arrayK.length/2; i++){
        if(!arrayK[i].isEmpty() && !arrayK[k-i].isEmpty())
        {
            for(Object index: arrayK[i])
                for(Object otherIndex: arrayK[k-i])
                    System.out.println("Numbers at indeces ["+index.toString()+", "+otherIndex.toString()+"] add up to "+k+".");
        }
    }

}