You've worked through array, string, and stack problems.
Hash maps turn searches into (Two Sum, Subarray Sum).
Stacks handle bracket matching and parentheses validation (Valid Parentheses, Minimum Remove).
Sorting puts overlapping data adjacent for single-pass merging (Merge Intervals).
Sliding window finds optimal substrings in (Longest Substring).
Two pointers rearrange arrays in-place (Move Zeroes).
Prefix/suffix decomposition handles "everything except current" queries (Product of Array Except Self).