LeetCode

LeetCode 196 note: it must be a DELETE, not a SELECT; keep only the smallest id per email and delete the rest

LeetCode 183 note: LEFT JOIN Orders, and customers with no matching order (o.id IS NULL) never ordered; NOT IN also works

LeetCode 182 note: group by email and keep groups with HAVING COUNT(*) > 1

LeetCode 181 note: self-join Employee on managerId = id to pair employees with managers, then compare salaries

LeetCode 175 note: people without an address must still appear, so LEFT JOIN, leaving city and state NULL when unmatched

LeetCode 26 note: overwrite in place with a write pointer for the next slot, writing only values different from the last one written

LeetCode 14 note: bound by the shortest string and check each position across all strings; the first mismatch gives the answer’s length

LeetCode 13 note: add each symbol’s value left to right, subtracting instead when it is smaller than the symbol to its right

LeetCode 9 note: negatives are never palindromes; without strings, reverse the second half of the digits and compare it with the first half

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

LeetCode 1 note: store seen numbers in a hash map while scanning and look up target - x; checking before storing prevents using one element twice