Intuition

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

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

You can return the answer in any order.

Example 1:

Input: nums = [2,7,11,15], target = 9 Output: [0,1] Explanation: Because nums[0] + nums[1] == 9, we return [0, 1]. Example 2:

Input: nums = [3,2,4], target = 6 Output: [1,2] Example 3:

Input: nums = [3,3], target = 6 Output: [0,1]

Constraints:

2 <= nums.length <= 104 -109 <= nums[i] <= 109 -109 <= target <= 109 Only one valid answer exists.

Approach

1. TwoSum using complementary

ref1: https://www.youtube.com/watch?v=Eipu0MqwS2s&ab_channel=TechTalk

ref2: https://www.youtube.com/watch?v=KLlXCFG5TnA&ab_channel=NeetCode

Complexity

• Time complexity: O(N)
• Space complexity: O(N)

Code 1

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        seen = {}
        i =0
        while i < len(nums):
            num = nums[i]
            comp = target - num
            if comp in seen:
                return [i, seen[comp]]
            seen[num] = i
            i += 1

Code 2

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        prevMap = {} #valu : index
        
        for i, n in enumerate(nums):
            diff = target - n
            if diff in prevMap:
                return [prevMap[diff], i]
            prevMap[n] = i
        return

ref: https://www.youtube.com/watch?v=KLlXCFG5TnA&ab_channel=NeetCode