Problem - Boats to Save People

People have weights. Each boat has a limit and holds at most $2$ people. Minimize boats used.

Example: people = [3,2,2,1], limit = $3$. Boats: (1,2), ($2$), ($3$). Total: $3$ boats. What is the greedy pairing strategy? Constraint: $n \le 5 \times 10^4$ and you need $O(n \log n)$ time using $O(1)$ space sorting. Naive assignment without sorting can waste boat capacity and use more boats than necessary.