Brainteaser

Combinatorial counting (pairs, ways to split), building the k-th permutation digit by digit, geometry (nearest point, law of cosines, triangle inequality), problems that become formulas after sorting or an observation, plus carry arithmetic, base conversion and fast exponentiation

LeetCode 3828 note: Alice can leave only the first or last element on her first move, guaranteeing max(first, last); Bob can likewise keep only an end, and one deletion changes at most one end, so the answer is max(first, last)

LeetCode 2745 note: AA and BB must alternate, using min(x, y) pairs plus one extra of the more common kind, and every AB fits in

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

LeetCode 292 note: whoever faces a multiple of 4 stones loses, so the first player wins when n % 4 != 0