1. Two Sum

Description

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9, return [0, 1].

Code:

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