花花酱 LeetCode 1754. Largest Merge Of Two Strings

You are given two strings word1 and word2. You want to construct a string merge in the following way: while either word1 or word2 are non-empty, choose one of the following options:

• If word1 is non-empty, append the first character in word1 to merge and delete it from word1.
  • For example, if word1 = "abc" and merge = "dv", then after choosing this operation, word1 = "bc" and merge = "dva".
• If word2 is non-empty, append the first character in word2 to merge and delete it from word2.
  • For example, if word2 = "abc" and merge = "", then after choosing this operation, word2 = "bc" and merge = "a".

Return the lexicographically largest merge you can construct.

A string a is lexicographically larger than a string b (of the same length) if in the first position where a and b differ, a has a character strictly larger than the corresponding character in b. For example, "abcd" is lexicographically larger than "abcc" because the first position they differ is at the fourth character, and d is greater than c.

Example 1:

Input: word1 = "cabaa", word2 = "bcaaa"
Output: "cbcabaaaaa"
Explanation: One way to get the lexicographically largest merge is:
- Take from word1: merge = "c", word1 = "abaa", word2 = "bcaaa"
- Take from word2: merge = "cb", word1 = "abaa", word2 = "caaa"
- Take from word2: merge = "cbc", word1 = "abaa", word2 = "aaa"
- Take from word1: merge = "cbca", word1 = "baa", word2 = "aaa"
- Take from word1: merge = "cbcab", word1 = "aa", word2 = "aaa"
- Append the remaining 5 a's from word1 and word2 at the end of merge.

Example 2:

Input: word1 = "abcabc", word2 = "abdcaba"
Output: "abdcabcabcaba"

Constraints:

• 1 <= word1.length, word2.length <= 3000
• word1 and word2 consist only of lowercase English letters.

Solution: Greedy

Always take a single char from the largest word. (NOT just the current char).

E.g.
ans = “”, w1 = “cabba”, w2 = “bcaaa”
w1 > w2, take from w1
ans = “c”, w1 = “abba”, w2 = “bcaaa”
w1 < w2, take from w2
ans = “cb”, w1 = “abba”, w2 = “caaa”
w1 < w2, take from w2
ans = “cbc”, w1 = “abba”, w2 = “aaa”
w1 > w2, take from w1. Note: both start with “a”, but we need to compare the entire word.
ans = “cbca”, w1 = “bba”, w2 = “aaa”
w1 > w2, take from w1
ans = “cbcab”, w1 = “ba”, w2 = “aaa”
…

Time complexity: O(min(m,n)^2)
Space complexity: O(1)