1. Two Sum

Difficulty: EASY

Problem description can be found here.

Approach

Basically use a HashMap. The key is to store the difference from each number in the array from the target.

If the difference exist, we basically found our solution.

Solution

function twoSum(nums: number[], target: number): number[] {
  const hash: Record<number, number> = {};
  const output: number[] = [];

  for (let i = 0; i < nums.length; i++) {
    const difference = target - nums[i];

    if (difference in hash) {
      output.push(i);
      output.push(hash[difference]);
    }

    hash[nums[i]] = i;
  }

  return output;
}