String

LeetCode 151 note: split() with no argument treats runs of spaces as one separator and drops leading and trailing spaces; reverse and join with single spaces

LeetCode 93 note: cut into exactly 4 pieces, each 1 to 3 digits long, valued 0 to 255, with no leading zero except 0 itself

LeetCode 647 note: expand around centers, counting one palindromic substring for each successful expansion

LeetCode 131 note: each level cuts a palindrome from the front of the remaining string, trying every length from 1 to the whole remainder

LeetCode 208 note: each node is a children dict plus an is_end flag; insert, search and startsWith all walk down character by character

LeetCode 981 note: store a (timestamp, value) list per key, with timestamps increasing; get binary searches the last timestamp <= the query

LeetCode 424 note: a variable sliding window where window length minus the top character count is the number of replacements; shrink when it exceeds k

LeetCode 79 note: backtrack from every cell, matching the next character in four directions with visited cells marked temporarily and restored; prune early with character counts

LeetCode 71 note: split on /, skip empty pieces and ., pop on .. (never above root), push other names, and join with /

LeetCode 49 note: use each string’s sorted form as a key; strings sharing a key form one anagram group

LeetCode 43 note: num1[i] × num2[j] lands at position i + j + 1; accumulate into an array of length n + m and carry at the end

LeetCode 5 note: expand around centers, using each position as an odd and an even center; with interval DP, iterate i from large to small