LeetCode 1. Two Sum java solution with HashMap

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].

使用HashMap存储不同的数据，比for循环的方式的时间复杂度低

public class Solution {    public int[] twoSum(int[] nums, int target) {        if (nums == null || nums.length == 0) {            return null;        }        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};             }            map.put(target - nums[i], i);        }        return null;    }}