📝 **Task:** Write `two_sum(nums, target)` returning `[i, j]` of two indices whose values sum to target (assume exactly one solution). Print `two_sum([2, 7, 11, 15], 9)`. 📋 Implement the function above. Test cases run automatically. 💡 **Hint:** Re-read the theory if you get stuck — the pattern is right there.