You've worked through array and string problems.
Hash maps for lookup (Two Sum)
Prefix/suffix products without division (Product of Array Except Self)
Sort + linear scan for intervals (Merge Intervals)
Two pointers from both ends (Trapping Rain Water)
Custom comparators for sorting (Reorder Log Files)
Expand-around-center for palindromes (Longest Palindromic Substring)
In-place compression (String Compression)
Chunk processing by digit groups (Integer to English Words)
Transpose + reverse for rotation (Rotate Image)
Boundary shrinking for spirals (Spiral Matrix)