• i. Prefix Sums and Difference Arrays: Prefix sums give $O(1)$ range queries on a static array.
• ii. Two Pointers and Sliding Window: Keep a window $[l,r]$ whose ends only move forward.
• iii. Monotonic Stack: A stack kept sorted answers “nearest element to the left/right that is smaller/greater” for every position in $O(n)$ total.
• iv. Classic Array Problems: Short problems with techniques that transfer:
• v. Subarray Aggregates and Mex: Two families of “over all subarrays” problems.