Easy

LeetCode 263 note: only positive numbers can be ugly; divide out every factor 2, 3 and 5 and check whether 1 remains

LeetCode 258 note: sum the digits repeatedly until one remains; the O(1) answer is the digital root 1 + (n - 1) % 9

LeetCode 257 note: DFS carrying the path string down, adding the whole path at each leaf

LeetCode 242 note: two strings are anagrams exactly when their character counts match; compare Counters or sorted strings

LeetCode 234 note: find the middle with fast and slow pointers, reverse the second half, and compare from both ends with O(1) extra space

LeetCode 232 note: two stacks, where in receives pushes and out is refilled from in only when empty, amortized O(1)

LeetCode 231 note: a power of two has exactly one set bit, and n & (n - 1) clears the lowest one, so the result is 0 exactly then; n must be positive

LeetCode 228 note: scan once, extending the current range when a number continues the previous one and opening a new range otherwise, then format

LeetCode 226 note: swap every node’s children, using a simultaneous assignment so the already-changed side is not reused

LeetCode 225 note: one queue suffices: after a push, rotate all earlier elements to the back so the new element sits at the front

LeetCode 219 note: a hash map stores each value’s last index; only the most recent occurrence needs a distance check

LeetCode 217 note: if converting to a set shortens the array, there is a duplicate