【Leetcode】Reverse Words in a String

最新推荐文章于 2025-08-10 20:50:18 发布

原创最新推荐文章于 2025-08-10 20:50:18 发布 · 218 阅读

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CC 4.0 BY-SA版权

文章标签：

#leetcode #Java #String #stringbuilder #reverse

本文介绍了一种用于将输入字符串中的单词进行反转的算法，并提供了两种不同的实现方式，一种使用split方法，另一种则通过递归实现。

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

For example,
Given s = "the sky is blue",
return "blue is sky the".

click to show clarification.

Clarification:

What constitutes a word?
A sequence of non-space characters constitutes a word.
Could the input string contain leading or trailing spaces?
Yes. However, your reversed string should not contain leading or trailing spaces.
How about multiple spaces between two words?
Reduce them to a single space in the reversed string.

Java:

重点就是用了spilt("\\s+")操作，然后考虑下边界情况：null," ",""

public class Solution {
   public String reverseWords(String s) {  
      if(s==null) return null;
      s=s.trim();
      if(s.length()==0) return "";
      
      String[] temp=s.split("\\s+");
      StringBuilder sb= new StringBuilder();
      int i=temp.length-1;
      while(i>=0)
      {
          sb.append(temp[i]);
          if(i!=0) sb.append(" ");
          i--;
      }
      return sb.toString();
    }  
}

Reference:

http://blog.youkuaiyun.com/linhuanmars/article/details/20982463

第一种没看懂，第二种：

public class Solution {
   public String reverseWords(String s) {  
      if(s==null) return null;
      s=s.trim();
      if(s.length()==0) return "";
      StringBuilder res= new StringBuilder();
      for(int i=s.length()-1;i>=0;i--)
      {
          if(s.charAt(i)!=s.length()&&s.charAt(i)==' '&&s.charAt(i)==s.charAt(i+1)) continue;
          res.append(s.charAt(i));
      }
      int left=0;
      int right=0;
      while(right<res.length())
      {
          while(right<res.length()&&res.charAt(right)!=' ')
          {
              right++;
          }
          int next=right+1;
          right--;
          while(left<right)
          {
              char temp=res.charAt(left);
              res.setCharAt(left++, res.charAt(right));
              res.setCharAt(right--,temp);
              
          }
          left=next;
          right=next;
          
      }
      return res.toString();
    }  
}

第一种递归：

public String reverseWords(String s) {
    s = s.trim();
    return helper(s,0).toString();
}
private StringBuilder helper(String s, int index)
{
    if(index>=s.length())
        return new StringBuilder(); 
    StringBuilder cur = new StringBuilder();
    int lastIndex = index;
    while(index < s.length() && s.charAt(index)!=' ')
    {
        cur.append(s.charAt(index++));
    }
    while(index < s.length() && s.charAt(index)==' ')
        index++;
    if(lastIndex == 0)
        return helper(s,index).append(cur);
    return helper(s,index).append(cur).append(' ');
}