1. Two Sum

Example:
Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

    public int[] twoSum(int[] nums, int target) {
        for (int i = 0; i < nums.length - 1; i++){
            for (int j = i+1; j < nums.length; j++){
                if (target == nums[i] + nums[j]) {
                    return new int[] {i,j};
                }
            }
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}