Medium

LeetCode 606 note: output in pre-order, keeping an empty () when only the left subtree is missing, and omitting the right parentheses when it is empty

LeetCode 237 note: without the previous node, copy the next node’s value over and delete the next node instead

LeetCode 235 note: go left when both p and q are smaller, right when both are larger; otherwise (split, or one is the node itself) this node is the LCA

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

LeetCode 7 note: remember the sign, reverse the digits of the absolute value, and return 0 when the result leaves the signed 32-bit range