var twoSum = function(nums, target) { let history = []; for(let i=0; i< nums.length; i++){ let diff = target - nums[i]; //I get the difference I need to find in history. let existIndex = history.indexOf(diff); //Get index if exist or -1 if(existIndex != -1) { return [existIndex, i]; //if exists, return index and current index. } history.push(nums[i]); } };