[leetcode #151] Reverse Words in a String

Problem

Given an input string s, reverse the order of the words.
A word is defined as a sequence of non-space characters. The words in s will be separated by at least one space.
Return a string of the words in reverse order concatenated by a single space.
Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.
Example 1:

Input: s = "the sky is blue"
Output: "blue is sky the"

Example 2:

Input: s = "  hello world  "
Output: "world hello"
Explanation: Your reversed string should not contain leading or trailing spaces.

Example 3:

Input: s = "a good   example"
Output: "example good a"
Explanation: You need to reduce multiple spaces between two words to a single space in the reversed string.

Example 4:

Input: s = "  Bob    Loves  Alice   "
Output: "Alice Loves Bob"

Example 5:

Input: s = "Alice does not even like bob"
Output: "bob like even not does Alice"

Constraints:

・ 1 <= s.length <= 104
・ s contains English letters (upper-case and lower-case), digits, and spaces ' '.
・ There is at least one word in s.

Idea

Javaでサポートされているstring libraryを使用すれば、この問題を解決できます.
まず、regexを使用してスペースを区切り文字として使用すると、文字列は自動的に区切ります.分離した文字列をstring builderに逆順に挿入し、文字列の最後にスペースを挿入します.最後にtrimを呼び出して前後の空間の空白を除去します.
libraryを使うのは簡単すぎて、問題に対する評価がよくありません.

忙しい中こんな簡単な問題があってよかった.

Solution

class Solution {
    public String reverseWords(String s) {
        String[] list = s.split("\\s+");
        StringBuilder strBuilder = new StringBuilder();

        for (int i=list.length-1; i >= 0; i--) {
            strBuilder.append(list[i]);
            strBuilder.append(" ");
        }

        return strBuilder.toString().trim();
    }
}

Reference

https://leetcode.com/problems/reverse-words-in-a-string/