字符串单词翻转总结几种常见方法

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

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

eg.
Given s = "the sky is blue",

return "blue is sky the".

1.直接使用split()方法：

private void reverseSentence(String sentence) {
	if(sentence==null)
		return;
	String[] str = sentence.split("\\s+");
	//String[] str = sentence.split(" ");
	for (int i = 0; i < str.length; i++) {
		System.out.print(str[i]+" ");		
	}
	System.out.println();
	StringBuffer sb = new StringBuffer();
	for (int i = str.length-1; i >=0; i--) {
		sb.append(str[i]+" ");
	}
	System.out.println(sb);
}

该方法无法处理输入样例：“”,因为输出会是“ ”(带有空格)。

2.不实用split()方法：

ArrayList<String> word = new ArrayList<String>();
    public String reverseWords(String s) {
        int wordNum = calWordNum(s);

        StringBuffer sb = new StringBuffer();
        for(int i = 0;i<wordNum;i++){
            sb.append(word.get(wordNum-i-1));
            if(i!=wordNum - 1){
                sb.append(" ");
            }
        }
        return sb.toString();
    }

    private int calWordNum(String s){
        int cursor = 0;
        int wordNum = 0;
        StringBuffer sb = new StringBuffer();
        for(int i = 0;i < s.length();i++)
        {
            if(i==0){//判断起始位置是否为空格
                if(s.charAt(i)==' '){
                    continue;
                }else if(s.length()!= 1){
                    sb.append(s.charAt(i));
                    cursor++;
                }else if(s.length()==1){
                    sb.append(s.charAt(i));
                    word.add(sb.toString());
                    wordNum++;
                }
            }else if(i==s.length() - 1){//判断结束位置是否为空格
                if(s.charAt(i)==' '){
                    if(cursor != 0){
                        wordNum++;
                        word.add(sb.toString());
                    }
                }else{
                    if(cursor==0){
                        wordNum++;
                        sb.append(s.charAt(i));
                        word.add(sb.toString());
                    }else{
                        sb.append(s.charAt(i));
                        word.add(sb.toString());
                        wordNum++;
                    }
                }
            }else{
                if(s.charAt(i)==' '){
                    if(cursor==0){
                        continue;
                    }else if(cursor!=0){
                        cursor = 0;
                        wordNum++;
                        word.add(sb.toString());
                        sb.delete(0,sb.length());
                    }
                }else{
                    sb.append(s.charAt(i));
                    cursor++;
                }
            }
        }
        return wordNum;
    }

参考： 点击打开链接

3.对split()方法进行改进：

public String reverseWords(String input) {
	    final StringBuilder stringBuilder = new StringBuilder();
	    for (final String part : input.split("\\s+")) {
	        if (!part.isEmpty()) {
	          if (stringBuilder.length() > 0) {
	            stringBuilder.insert(0, " ");
	          }
	          stringBuilder.insert(0, part);
	        }
	    }
	    return stringBuilder.toString();
	    }

参考：点击打开链接

注：2,3两种方法在leetcode上均能通过。