Python: Two Sum Problem

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

The below python code will return the first solution found, and assumes that no element appears twice.

def twoSum(nums, target):
    """
    :type nums: List[int]
    :type target: int
    :rtype: List[int]
    """
    index_map = {}
    for i in range(len(nums)):
        num = nums[i]
        pair = target - num
        if pair in index_map:
            return [index_map[pair], i]
        index_map[num] = i
    return None

nums = [1,4,8,3,2,9,15]
target = 13
print("Nums: ", nums)
print("Target: ", target)
print("Solution: ", twoSum(nums, target))

print()
nums = [2,7,11,15]
target = 9
print("Nums: ", nums)
print("Target: ", target)
print("Solution: ", twoSum(nums, target))

Output:

Nums:  [1, 4, 8, 3, 2, 9, 15]
Target:  13
Solution:  [1, 5]

Nums:  [2, 7, 11, 15]
Target:  9
Solution:  [0, 1]

Hope it helps.