Trigger: “Sorted Array” + Finding a “Sum” + $O(1)$ Space constraint. Mechanical Approach: Opposite-Directional Two Pointers.

• left at start (0), right at end ($N-1$).
• If sum < target: we need a bigger number $\rightarrow$ left += 1.
• If sum > target: we need a smaller number $\rightarrow$ right -= 1.
• If sum == target: Return indices.

System/Engineering Value: - Replaces $O(N)$ HashMap memory with $O(1)$ pointer memory. Crucial for massive, sorted datasets that cannot fit into RAM (e.g., streaming large log files from disk where we only hold pointer offsets).

Blindspot: - The problem explicitly asks for 1-based indices. Return [left + 1, right + 1].