Problem:

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

Each input has exactly one solution, and you may not use the same element twice.

brute force - O(n²) time

for i in range(len(nums)):
    # loop through i+1 -> end of the array
    for j in range(i+1, len(nums)):
        if nums[i] == (target - nums[j]):
            return [i,j]

# fail case   
return [-1, -1]

O(n) space, O(n) time - 2 passes

numsMap = {}

# loop through list and add value:index to map  
for i in range(len(nums)):
    numsMap[nums[i]] = i

# loop through list again to find pair
for i in range(len(nums)):
    difference = target - nums[i]

    # check if the difference is in the map
    if difference in numsMap and (numsMap[difference] != i):
        return [numsMap[difference], i]
      
# fail case    
return [-1, -1]

O(n) space and time - 1 pass

numsMap = {}

#enumerate through indexes and values in nums
for i, v in enumerate(nums):
    difference = target - v

    # check if the difference is in the map
    if difference in numsMap:
        return [numsMap[difference], i]

    # add value:index to map 
    numsMap[v] = i

# fail case    
return [-1, -1]