---
layout: page
title:  Two Sum
last_solved: 2020-08-04
category: array, hashtable
leetcode_url: https://https://leetcode.com/problems/two-sum
status: Solved
---

### Problem

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

{% highlight text %}

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

{% endhighlight %}

### Brute Force Solution

{% highlight python %}

nums = [2, 7, 11, 15]
target = 26


def twoSum(nums, target):

    length = len(nums)
    for i in range(0, length):
        for j in range(i+1, length):

            if nums[i] + nums[j] == target:

                return (i, j)
                break

            j=j+1


res = twoSum(nums, target)
print res


{% endhighlight %}

### OnePass HashTable SOlution

This is a common interview problem. Just memorize the technique.

{% highlight python %}

def twoSum(nums, target):

        seen = {}
        
        for i, v in enumerate(nums):
            
            remaining = target - v
            
            # if seen, return indexes
            if remaining in seen:
                return [seen[remaining], i]
            
            # else, add it to seen hashtable
            else:
                seen[v] = i
        
        
        return []
{% endhighlight %}

____________

Another way of writing it, not as nice. Use enumerate.

{% highlight python %}

# not using enumerate
def twoSum(self, nums: List[int], target: int) -> List[int]:
    
    seen = {}
    
    for i in range(len(nums)):
        
        remaining = target-nums[i]
        
        if nums[i] in seen:
            return [i, seen[nums[i]]]
        else:
            seen[remaining] = i

# using enumerate
def twoSum(self, nums: List[int], target: int) -> List[int]:
    
    seen = {}
    
    for i, v in enumerate(nums):
        
        remaining = target-v
        
        if nums[i] in seen:
            return [i, seen[v]]
        else:
            seen[remaining] = i
        

{% endhighlight %}