Two Sum

https://leetcode.com/problems/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.

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]

Solution

That is the base problem. Based on it, you can solve 3 sum and 4 sum.

Sure, sure, you can solve it with a brute force algorithm. For example, two cycles and that is it.

But, we will try to speed up our algorithm. How can we do it? Let's add a hash map and put our elements in it. And... and that is it. Based on elements in the map, we can calculate a needed sum.

  public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> map = new HashMap<>();
     
    for(int i = 0; i < nums.length; i++){
      if(map.containsKey(nums[i])){
        return new int [] {map.get(nums[i]), i};
      } else {
        map.put(target - nums[i], i);
      }
    }
    return null;
  }