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# 151.Reverse-Words-in-a-String

## 151. Reverse Words in a String

## 题目地址

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

## 题目描述

```
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.
```

## 代码

### Approach 1: Built-in Split + Reverse

* Time complexity & Space complexity O(*N*)

```java
class Solution {
  public String reverseWords(String s) {
        s = s.trim();
    List<String> wordList = Arrays.asList(s.split("\\s+"));
    Collections.reverse(wordList);
    return String.join(" ", wordList);
  }
}
```

### Approach #2 Reverse the Whole String and then Reverse each word

```java
class Solution {
  public String reverseWords(String s) {
        StringBuilder sb = trimSpace(s);

    reverse(sb, 0, sb.length() - 1);

    reverseEachWord(sb);

    return sb.toString();
  }

  public void reverse(StringBuilder sb, int left, int right) {
    while (left < right) {
      char tmp = sb.charAt(left);
      sb.setCharAt(left++, sb.charAt(right));
      sb.setCharAt(right--, tmp);
    }
  }

  public void reverseEachWord(StringBuilder sb) {
    int n = sb.length();
    int start = 0, end = 0;

    while (start < n) {
      while (end < n && sb.charAt(end) != ' ') {
        end++;
      }
      reverse(sb, start, end - 1);

      start = end + 1;
      end++;
    }
  }


  public StringBuilder trimSpace(String s) {
    int left = 0, right = s.length() - 1;
    while (left <= right && s.charAt(left) == ' ') {
      left++;
    }
    while (left <= right && s.charAt(right) == ' ') {
      right--;
    }

    StringBuilder sb = new StringBuilder();
    while (left <= right) {
      char c = s.charAt(left);
      if (c != ' ') {
        sb.append(c);
      } else if (sb.charAt(sb.length() - 1) != ' ') {
        sb.append(c);
      }
      left++;
    }

    return sb;
  }

}
```