说明

算法：Reverse Words in a String
LeetCode地址：https://leetcode.com/problems/reverse-words-in-a-string/

题目：
Given an input string, reverse the string word by word.

Example 1:

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

Example 2:

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

Example 3:

Input: "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.

Note:

A word is defined as a sequence of non-space characters.
Input string may contain leading or trailing spaces. However, your reversed string should not contain leading or trailing spaces.
You need to reduce multiple spaces between two words to a single space in the reversed string.

Follow up:

For C programmers, try to solve it in-place in O(1) extra space.

解题思路1

翻转字符串里的单词， split() 先把单词拆解为wordsArray，倒序把word拼接即可。
时间复杂度为 O(N)；空间复杂度为O(N).

代码实现1

import java.util.Arrays;public class ReverseWordsInAString {public String reverseWords(String s) {s = s.trim();if (s.length() == 0) {return s;}String blankString = " ";String[] wordsArray = s.split(blankString);StringBuilder sb = new StringBuilder();for (int i = wordsArray.length - 1; i >= 0; i--) {String word =  wordsArray[i];word = word.trim();if (word.length() > 0) {sb.append(word).append(blankString);}}return sb.toString().trim();}public static void main(String[] args) {String input = "the sky is blue";ReverseWordsInAString obj = new ReverseWordsInAString();System.out.println("Input: \"" + input + "\"\n" + "Output: \"" + obj.reverseWords(input) + "\"") ;String input1 = "  hello world!  ";System.out.println("Input: \"" + input1 + "\"\n" + "Output: \"" + obj.reverseWords(input1) + "\"") ;String input2 = "a good   example";System.out.println("Input: \"" + input2 + "\"\n" + "Output: \"" + obj.reverseWords(input2) + "\"") ;}
}

运行结果1

Input: "the sky is blue"
Output: "blue is sky the"
Input: "  hello world!  "
Output: "world! hello"
Input: "a good   example"
Output: "example good a"

代码执行效率1

Runtime: 2 ms, faster than 92.72% of Java online submissions for Reverse Words in a String.
Memory Usage: 38.5 MB, less than 66.81% of Java online submissions for Reverse Words in a String.

解题思路2

降低空间复杂度，把字符串转为字符数组 > 字符数组原地翻转 > 找到单词在word内部翻转回来 > 清理多余的空字符并替换掉前面的字符 > 新建字符串，截取到前面有效字符。
时间复杂度为 O(N)，空间复杂度也为O(N), 但是前面的系数明显会小于上面的。

代码实现2

import java.util.Arrays;public class ReverseWordsInAString {public String reverseWordsWithLessSpace(String s) {char[] chars = s.toCharArray();int len = chars.length;// step 1. reverse the whole stringsubReverse(chars, 0, len - 1);// step 2. reverse each wordsubReverseWords(chars, len);// step 3. clean up spacesreturn subCleanBlankChar(chars, len);}// trim leading, trailing and multiple spacespublic String subCleanBlankChar(char[] chars, int len) {int i = 0, j = 0;char blankChar = ' ';while (j < len) {// skip spaceswhile (j < len && chars[j] == blankChar) {j++;}// keep non spaceswhile (j < len && chars[j] != blankChar) {chars[i++] = chars[j++];}// skip spaceswhile (j < len && chars[j] == blankChar) {j++;}// keep only one spaceif (j < len) {chars[i++] = blankChar;}}return new String(chars).substring(0, i);}public void subReverseWords(char[] chars, int len) {char blankChar = ' ';int i = 0, j = 0;while (j < len) {// skip spaceswhile (j < len && chars[j] == blankChar) {j++;}i = j;// skip non spaceswhile (j < len && chars[j] != blankChar) {j++;}// reverse the wordsubReverse(chars, i, j - 1);}}// reverse a[] from a[i] to a[j]public void subReverse(char[] chars, int startIndex, int endIndex) {while (startIndex < endIndex) {char temp = chars[startIndex];chars[startIndex] = chars[endIndex];chars[endIndex] = temp;startIndex++;endIndex--;}}public static void main(String[] args) {String input = "the sky is blue";ReverseWordsInAString obj = new ReverseWordsInAString();System.out.println("Input: \"" + input + "\"\n" + "Output: \"" + obj.reverseWordsWithLessSpace(input) + "\"") ;String input1 = "  hello world!  ";System.out.println("Input: \"" + input1 + "\"\n" + "Output: \"" + obj.reverseWordsWithLessSpace(input1) + "\"") ;String input2 = "a good   example";System.out.println("Input: \"" + input2 + "\"\n" + "Output: \"" + obj.reverseWordsWithLessSpace(input2) + "\"") ;}
}

运行结果2

Input: "the sky is blue"
Output: "blue is sky the"
Input: "  hello world!  "
Output: "world! hello"
Input: "a good   example"
Output: "example good a"

代码执行效率2

Runtime: 4 ms, faster than 71.55% of Java online submissions for Reverse Words in a String.
Memory Usage: 37.7 MB, less than 83.83% of Java online submissions for Reverse Words in a String.

总结

翻转字符串里的单词，第一种方案：拆分单词数组，倒序拼接最直接，也是最快的。第二种方案：减少空间复杂度，用到字符数组原地排序的技巧。

代码下载：
https://github.com/zgpeace/awesome-java-leetcode/blob/master/code/LeetCode/src/popular/ReverseWordsInAString.java

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算法:Reverse Words in a String(翻转字符串里的单词)

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