Math

LeetCode 50 note: fast exponentiation, x^n = (x²)^(n/2) with an extra x for odd n; for negative n compute 1 / x^(-n)

LeetCode 264 note: three pointers mark the next ugly numbers to multiply by 2, 3 and 5; take the minimum each time and advance every pointer that produced it

LeetCode 227 note: read whole numbers, and at each operator apply the previous one: + and - push the number (negated for -), * and / combine with the top

LeetCode 204 note: the sieve of Eratosthenes, crossing out multiples of each prime p starting at p², with the outer loop only up to √n

LeetCode 189 note: take k %= n, then reverse three times: the whole array, the first k, and the last n - k, with O(1) extra space

LeetCode 48 note: rotating 90 degrees clockwise is a transpose followed by reversing each row, both in place

LeetCode 973 note: as in Top-K, keep a size-k max-heap (negated distances) holding the k closest points

LeetCode 43 note: num1[i] × num2[j] lands at position i + j + 1; accumulate into an array of length n + m and carry at the end

LeetCode 1137 note: like Fibonacci, roll three variables; watch the base values T0 = 0, T1 = T2 = 1 and the loop range

LeetCode 1103 note: simulate the rounds, giving i + 1 candies on the i-th turn, and hand all the rest to the current person when short

LeetCode 1030 note: list every coordinate and sort by Manhattan distance to the center; return coordinates, not distances

LeetCode 1025 note: dp[i] holds when some divisor x of i makes dp[i - x] a loss; observing the pattern, the answer is simply whether n is even