Given an array of integers, return indices of the two numbers such that they add up to a specific target.
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:
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 Given nums = [2, 7, 11, 15], target = 9, Because nums[0] + nums[1] = 2 + 7 = 9, return [0, 1].

Solution
From Ang’s:
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 public int[] twoSum(int[] nums, int target) { if(nums == null){ throw new IllegalArgumentException("Nums should not null"); } int count = nums.length; if(count == 0){ throw new IllegalArgumentException("Nums should not empty"); } int i, j; for(i = 0; i<count; i++){ for(j = i+1; j<count; j++){ if(nums[i] + nums[j] == target){ return new int[] { i, j }; } } } throw new IllegalArgumentException("No two sum solution"); }

The better one (O(n)):
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 public int[] twoSum(int[] nums, int target) { Map<Integer, Integer> map = new HashMap<>(); for (int i = 0; i < nums.length; i++) { int complement = target  nums[i]; if (map.containsKey(complement)) { return new int[] { map.get(complement), i }; } map.put(nums[i], i); } throw new IllegalArgumentException("No two sum solution"); }
