Build the canonical Two Sum: `two_sum(nums, target)` returns the two indices summing to target (or `[-1, -1]` if no pair): - Use a dict `seen` mapping value → index, so each scan is O(1) lookup. - Return indices in the order you found them (smaller index first). - No pair → `[-1, -1]` (never raise). - Don't use the SAME index twice (e.g. for `[3], target=6`, do NOT return `[0,0]`). LeetCode #1 — the most-asked interview question on Earth.