Problem Statement

1. 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.

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

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

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

Intuition

Maintain a Hash map to store the complement of each number in the array. For each number, check if the complement value is present in the map.

If present, return the index of current value and complement value. If not present, calculate the complement and store it in the hash map along with the index.

Visualisation

Code

class Solution {
    public int[] twoSum(int[] nums, int target) {
        var cache = new HashMap<Integer, Integer>();
        for(var i=0; i<nums.length; i++) {
            if(cache.containsKey(nums[i])) {
                return new int[]{cache.get(nums[i]), i};
            }
            cache.put(target - nums[i], i);
        }
        return new int[]{};
    }
}

Learning

This problem expects us to know how to:

• Iterate over an array

• Use hashmap to store and retrieve values in constant time O(1)