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Two sum

  Given an array of integers   nums  and an integer   target , return   indices of the two numbers such that they add up to  target . You may assume that each input would have  exactly  one solution , and you may not use the  same  element twice. You can return the answer in any order.   Example 1: Input: nums = [2,7,11,15], target = 9 Output: [0,1] Explanation: Because nums[0] + nums[1] == 9, we return [0, 1]. Example 2: Input: nums = [3,2,4], target = 6 Output: [1,2] Example 3: Input: nums = [3,3], target = 6 Output: [0,1]   Constraints: 2 <= nums.length <= 10 4 -10 9 <= nums[i] <= 10 9 -10 9 <= target <= 10 9 Only one valid answer exists.   Follow-up:  Can you come up with an algorithm that is less than  O(n 2 )  time complexity? public class Solution {     public int [] TwoSum ( int [] nums , int target ) {         int [] answer = new int [ ...