One essential drill per topic — the best starting point for each category.
#127 — Unique Paths
#100 — Flood fill
#90 — Diameter of binary tree
#1 — Reverse array in-place
#76 — Letter combinations of phone
#51 — Binary search template
#83 — Count set bits
#63 — Climbing stairs
#42 — DFS iterative
#71 — Best time to buy/sell stock
#23 — Two sum (unsorted)
#57 — Min-heap push and pop
#114 — Meeting Rooms
#9 — Insert at position k
#120 — Happy Number
#47 — Fibonacci (memoized)
#101 — Best Time to Buy and Sell Stock
#29 — Insertion sort
#19 — Evaluate postfix expression
#34 — BST insert
#107 — Implement Trie / Prefix Tree
#126 — Remove Duplicates from Sorted Array