1. Two Sum

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].

Solution:

public class Solution {    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;    }}