Recursion

LeetCode 326 note: divide by 3 while possible and check for 1; without loops, n is a power of three exactly when it divides 3^19

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 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 206 note: three pointers prev / cur / nxt; save the next node, then point cur.next back at prev

LeetCode 203 note: the head may be deleted too, so a dummy head turns every deletion into deleting cur.next; do not advance after deleting

LeetCode 2 note: digits are stored in reverse, so add from the head digit by digit with a carry, looping until both lists end and no carry remains