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：Hashmap

Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int result[] = new int[2];
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        for(int i = 0; i < nums.length; i++) {
            if(map.containsKey(target - nums[i])) {
                result[0] = map.get(target - nums[i]);
                result[1] = i;
                return result;
            }
            map.put(nums[i], i);
        }
        return result;
    }
}