花花酱 LeetCode 1163. Last Substring in Lexicographical Order

Given a string s, return the last substring of s in lexicographical order.

Example 1:

Input: "abab"
Output: "bab"
Explanation: The substrings are ["a", "ab", "aba", "abab", "b", "ba", "bab"]. The lexicographically maximum substring is "bab".

Example 2:

Input: "leetcode"
Output: "tcode"

Note:

1. 1 <= s.length <= 10^5
2. s contains only lowercase English letters.

Key observation: The last substring must be a suffix of the original string, can’t a substring in the middle since we can always extend it.
e.g. leetcode -> tcode, can’t be “t”, “tc”, “tco”, “tcod”

Solution 1: Brute Force

Try all possible suffixes.
Time complexity: O(n^2)
Space complexity: O(1)

Solution 2: Keep max and compare with candidates

Find the first largest letter as a starting point, whenever there is a same letter, keep it as a candidate and compare with the current best. If the later is larger, take over the current best.

e.g. “acbacbc”

“c” > “a”, the first “c” becomes the best.
“c” = “c”, the second “c” becomes a candidate
starting compare best and candidate.
“cb” = “cb”
“cba” < “cbc”, cand_i is the new best.

Time complexity: O(n)
Space complexity: O(1)