EasyLeetCode #1Arrays & Hashing

Two Sum

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.

Constraints

2 <= nums.length <= 10^4, -10^9 <= nums[i] <= 10^9, -10^9 <= target <= 10^9

Examples

Input: nums = [2,7,11,15], target = 9

Output: [0,1]

Because nums[0] + nums[1] == 9, we return [0, 1].

Solution

Approach

Use a hash map to store each number and its index. For each number, check if (target - num) exists in the map. If it does, return both indices.

def twoSum(nums, target):
    seen = {}
    for i, num in enumerate(nums):
        complement = target - num
        if complement in seen:
            return [seen[complement], i]
        seen[num] = i
    return []

Complexity

Time:O(n)

Space:O(n)

Hints

1.Think about what data structure allows O(1) lookup
2.For each element, what value are you looking for?
3.Can you store elements as you iterate?

Asked at

Google