def twoSum(nums, target): 
    index = {} # create a dictionary
    for i in range(len(nums)):
        if (target-nums[i]) in index: # pass each value until its' matching pair is found
            return [index[target-nums[i]], i]
        index[nums[i]] = i # store values of non-matching pair
        
print(twoSum([2,7,11,15], 9))
# print(twoSum([3,3], 6))