Medium

LeetCode 394 note: on [ push the current string and count, on ] pop and combine; counts can have several digits

LeetCode 2300 note: sort potions, bisect a threshold per spell, and compute the threshold with integer ceil to avoid float error

LeetCode 3703 note: repeated replace is O(n^2); a stack of (character, run length) removes k ( plus k ) as soon as they line up

LeetCode 3689 note: the same subarray may be chosen repeatedly, and the whole array’s max minus min is the best single value, so choose it k times

LeetCode 2294 note: subsequences ignore order, so sort and extend each group from its minimum until a value exceeds min + k

LeetCode 1493 note: find the longest window with at most one 0; since one element must be deleted, the answer is the window length minus 1

LeetCode 3195 note: track the topmost, bottommost, leftmost and rightmost 1; the area is height times width

LeetCode 2131 note: pair each word with its reverse on both sides; self-palindromes also pair with each other, and only one leftover can sit in the middle

LeetCode 3649 note: simplify the condition to “larger absolute value <= 2 × smaller absolute value”, then count with two pointers over sorted absolute values

LeetCode 3648 note: each sensor covers a (2k + 1) × (2k + 1) square, so take a ceiling division in each direction and multiply

Solving the Online Stock Span problem using monotonic stack approach

Solving the Car Fleet problem using stack-based approach