Hard

LeetCode 2141 note: a battery longer than the average can only serve one computer, so remove it with that computer and average the rest, using strict greater-than

LeetCode 902 note: digit DP with state (pos, tight, started); before starting, a position can be skipped, representing numbers with fewer digits than n

LeetCode 3548 note: try each horizontal cut (transpose for vertical); the difference must equal a cell on the larger side, and a one-row or one-column side can only drop an end cell, while any cell works otherwise

LeetCode 980 note: grid backtracking that marks visited cells in place and checks at the end that every empty cell was covered

LeetCode 2421 note: add nodes in increasing value order and count same-value nodes per Union-Find group

LeetCode 2382 note: work backward so removals become additions; the Union-Find root stores the segment sum, and the maximum only grows

LeetCode 2355 note: ending at i, each shelf to the left takes one fewer book until a bottleneck shelf j with too few books; a monotonic stack finds j, and dp[i] = dp[j] + an arithmetic series

LeetCode note for Shortest Common Supersequence, rebuilt from the original learning note

LeetCode note for Minimum Insertion Steps to Make a String Palindrome, rebuilt from the original learning note

LeetCode note for Distinct Subsequences, rebuilt from the original learning note

LeetCode 3989 note: the kept columns form a subsequence constrained only between neighbors; an LIS-style DP with dp[j] = the longest valid sequence ending at column j

LeetCode 726 note: tokenize into elements, numbers, and parentheses; handle parentheses with a stack, with a default multiplier of 1