func twoSum(nums []int, target int) []int {
	dict := make(map[int]int) // num => idx

	for i := 0; i < len(nums); i++ {
		curr := nums[i]
		other := target - curr

		if idx, ok := dict[other]; ok {
			return []int{i, idx}
		} else {
			dict[curr] = i
		}
	}
	return nil
}