Interview Questions

Two sum problem

The two sum problem is a common interview question, and it is a variation of the subset sum problem. There is a popular dynamic programming solution for the subset sum problem, but for the two sum problem we can actually write an algorithm that runs in O(*n*) time. The challenge is to find all the pairs of two integers in an unsorted array that sum up to a given S.
## Naive solution

A naive approach to this problem would be to loop through each number and then loop again through the array looking for a pair that sums to S. The running time for the below solution would be O(*n*^{2}) because in the worst case we are looping through the array twice to find a pair.
## Faster solution

We can write a faster algorithm that will find pairs that sum to S in linear time. The algorithm below makes use of hash tables which have a constant lookup time. As we pass through each element in the array, we check to see if S minus the current element exists in the hash table. We only need to loop through the array once, resulting in a running time of O(*n*) since each lookup and insertion in a hash table is O(1).
## Example

If the array is: [4, 5, 1, 8] and the sum is 6 the algorithm would proceed with the steps below:
## Code for faster solution

## Sources

http://www.careercup.com/question?id=15206700
http://www.glassdoor.com/Interview/Given-a-sorted-array-write-a-program-to-decide-if-two-elements-sum-up-to-a-third-QTN_242990.htm
http://www.glassdoor.com/Interview/Given-a-sum-find-two-numbers-in-an-array-with-that-sum-QTN_864790.htm

For example, if the array is [3, 5, 2, -4, 8, 11] and the sum is 7, your program should return [[11, -4], [2, 5]] because 11 + -4 = 7 and 2 + 5 = 7.

// our two sum function which will return // all pairs in the array that sum up to S function twoSum(arr, S) { var sums = []; // check each element in array for (var i = 0; i < arr.length; i++) { // check each other element in the array for (var j = i + 1; j < arr.length; j++) { // determine if these two elements sum to S if (arr[i] + arr[j] === S) { sums.push([arr[i], arr[j]]); } } } // return all pairs of integers that sum to S return sums; } twoSum([3, 5, 2, -4, 8, 11], 7);

# our two sum function which will return # all pairs in the list that sum up to S def twoSum(arr, S): sums = [] # check each element in array for i in range(0, len(arr)): # check each other element in the array for j in range(i+1, len(arr)): # determine if these two elements sum to S if (arr[i] + arr[j] == S): sums.append([arr[i], arr[j]]) # return all pairs of integers that sum to S return sums print twoSum([3, 5, 2, -4, 8, 11], 7)

(1) The hash table is initially empty and the first element in the array is 4. We simply put 4 into the hash table.
(2) The next element is 5. We check to see if the sum minus the current element exists in the hash table. 6 - 5 = 1 does not exist in the hash table. So add 5 to the hash table.
(3) The next element is 1. We check to see if the sum minus the current element exists in the hash table. 6 - 1 = 5 does exist in the hash table so we found a pair!

// our two sum function which will return // all pairs in the array that sum up to S function twoSum(arr, S) { var sums = []; var hashTable = {}; // check each element in array for (var i = 0; i < arr.length; i++) { // calculate S - current element var sumMinusElement = S - arr[i]; // check if this number exists in hash table // if so then we found a pair of numbers that sum to S if (hashTable[sumMinusElement.toString()] !== undefined) { sums.push([arr[i], sumMinusElement]); } // add the current number to the hash table hashTable[arr[i].toString()] = arr[i]; } // return all pairs of integers that sum to S return sums; } twoSum([3, 5, 2, -4, 8, 11], 7);

# our two sum function which will return # all pairs in the list that sum up to S def twoSum(arr, S): sums = [] hashTable = {} # check each element in array for i in range(0, len(arr)): # calculate S minus current element sumMinusElement = S - arr[i] # check if this number exists in hash table # if so then we found a pair of numbers that sum to S if sumMinusElement in hashTable: sums.append([arr[i], sumMinusElement]) # add the current number to the hash table hashTable[arr[i]] = arr[i] # return all pairs of integers that sum to S return sums print twoSum([3, 5, 2, -4, 8, 11], 7)

mrdaniel
published this on 11/9/15 **|**

17

why do you have to convert, .toString()?

df5890

commented on 04/08/16

6

@df5890, in JavaScript hash tables, the keys cannot be integers. JavaScript will automatically convert the number to a string, but I just did it explicitly in the code above.
http://stackoverflow.com/questions/2002923/javascript-using-integer-as-key-in-associative-array

mrdaniel

commented on 04/13/16

3

what if there are duplicates in the given array?

jayceeyili

commented on 09/27/16

2

This solution would be better using a set over a hashTable.

dylanshine

commented on 08/27/18

1

How do i get the brackets to show up in my browser, when I run the code it shows up like this 5,2,-4,11, instead of
[[5,2],[-4,11]]

jcapra

commented on 10/13/16

1

Can you guys give code/comments for the ArrayAdditionI version of this classic problem, ie. returns true if there is a subset of any number of elements (not just two) adding up to the largest element? The solutions provided don't have comments for the most part. Thanks.

rhodesd

commented on 01/24/16

-7

This actually doesn't even correctly solve the challenge. It returns an empty array

adrian08

commented on 04/07/17

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