This problem was recently asked by Google and is a classic interview question often referred to as “Two Sum”.
The Problem
Given a list of numbers and a number k, return whether any two numbers from the list add up to k.
For example, given [10, 15, 3, 7] and k of 17, return true since 10 + 7 is 17.
Bonus: Can you do this in one pass?
Solution 1: Brute Force (The “Bad” Way)
The most intuitive approach is to check every pair of numbers. We can loop through the list twice.
def add_up_to_k_v1(mylist, k):
# This checks every combination
ans = [x + y == k for x in mylist for y in mylist]
return sum(ans) > 0
Complexity Analysis: * Time Complexity: O(N²) because of the nested iteration (or list comprehension). * Space Complexity: O(N²) because we are creating a list of boolean results for every pair.
This is not efficient for large lists.
Solution 2: Using a Set (The Efficient Way)
We can improve this significantly by using a set (hash map). As we iterate through the list, we can check if the “complement” (the number we need to reach k) has already been seen.
If we are at number num, we need to find k - num. If k - num exists in our set of seen numbers, we have found a pair!
def add_up_to_k_v2(mylist, k):
seen = set()
for num in mylist:
target = k - num
if target in seen:
return True
seen.add(num)
return False
# Test cases
mylist = [10, 15, 3, 7]
k = 17
assert add_up_to_k_v2(mylist, k) == True, 'Test Failed'
print("Test Passed!")
Complexity Analysis: * Time Complexity: O(N). We iterate through the list once. Set lookups are O(1) on average. * Space Complexity: O(N). In the worst case, we store all numbers in the set.
This “one pass” solution is much faster and is the expected answer in a coding interview.
Written by
Abdur-Rahmaan Janhangeer
Chef
Python author of 7+ years having worked for Python companies around the world
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