Find all pairs of integers within an array which sum to a specified value

Question

Design an algorithm to find all pairs of integers within an array which sum to a specified value.

I have tried this problem using a hash

Answer 1

Hashtable solution, in Ruby (quite straightforward to understand):

value = 100
pairs = [1,99,5,95]
hash_of_pairs = {}

pairs.map! do |pair|
  # Adds to hashtable the pair
  hash_of_pairs[pair] = pair

  # Finds the value the pair needs
  new_pair = hash_of_pairs[value - pair]

  # Returns the pair whenever the pair exists, or nil
  [new_pair, pair] if !new_pair.nil?
end.compact! # Cleans up the array, removing all nil values

print pairs # [[1,99], [5,95]]

Answer 2

Implemented in Python 2.7:

import itertools
list = [1, 1, 2, 3, 4, 5,]
uniquelist = set(list)
targetsum = 5
for n in itertools.combinations(uniquelist, 2):
    if n[0] + n[1] == targetsum:
    print str(n[0]) + " + " + str(n[1])

Output:

1 + 4
2 + 3

Answer 3

How about sorting the array, then marching in from both ends?

Answer 4

#include <iostream>
using namespace std;

#define MAX 15

int main()
{
 int array[MAX] = {-12,-6,-4,-2,0,1,2,4,6,7,8,12,13,20,24};
 const int find_sum = 0;
 int max_index = MAX - 1;
 int min_index = 0;
 while(min_index < max_index)
 {
  if(array[min_index] + array[max_index-min_index] == find_sum)
  {
   cout << array[min_index] << " & " << array[max_index-min_index] << " Matched" << endl;
   return 0;
  }
  if(array[min_index]+array[max_index-min_index] < find_sum)
  {
   min_index++;
   //max_index++;
  }
  if(array[min_index]+array[max_index-min_index] > find_sum)
  {
   max_index--;
  }
 }
 cout << "NO MATCH" << endl;
 return 0;
}
//-12 & 12 matched

Answer 5

Assume required sum = R

1. sort the array
2. for each number in the array A(n), do a binary search to find the number A(x) such that A(n) + A(x) = R

Answer 6

#python 3.x
def sum_pairs(list_data, number):    
    list_data.sort()
    left = 0
    right = len(list_data)-1
    pairs = []
    while left < right:        
        if list_data[left]+list_data[right] == number:
            find_pairs = [list_data[left], list_data[right]]
            pairs.append(find_pairs)
            right = right-1
        elif list_data[left]+list_data[right] < number:
            left = left+1
        else:
            right = right-1
    return pairs