Easy

LeetCode 2269 note: convert the number to a string and check every length-k substring; 0 cannot be a divisor

LeetCode 2200 note: extend each key k positions both ways, and keep a pointer to how far output has gone so overlapping ranges are not emitted twice

LeetCode 3794 note: one line with slicing; a hand-written two-pointer version must convert the string to a list first

LeetCode 584 note: referee_id != 2 misses NULLs, since comparing with NULL is unknown; add IS NULL explicitly

LeetCode 1757 note: both conditions must hold, so filter with WHERE … AND …

LeetCode 1281 note: walk the digits once, accumulating product and sum, and return product minus sum

LeetCode 1512 note: count while scanning; each number pairs with every earlier equal number, so add its current count to the answer

LeetCode 1486 note: generate start + 2i by definition and XOR them together

LeetCode 2469 note: apply the formulas directly, Kelvin = C + 273.15 and Fahrenheit = C × 1.8 + 32

LeetCode 2413 note: lcm(2, n), which is n when n is even and 2n when odd

LeetCode 2236 note: a warm-up, plus practice with C++ pointer access

LeetCode 2235 note: return the sum; the C++ version once failed to compile from a missing semicolon