Problem Statement
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.
Approach
Use a hash map to store each number and its index as we iterate. For each number, check if target - num exists in the map.
Why This Works
- ▸Hash map provides O(1) lookup
- ▸Single pass through the array
- ▸Store complement relationship
Complexity Analysis
| Metric | Value |
|---|---|
| Time | O(n) |
| Space | O(n) |
Examples
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.
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