剑指 Offer 58 - I. 翻转单词顺序

剑指 Offer 58 - I. 翻转单词顺序 - 力扣（LeetCode） (leetcode-cn.com)

输入一个英文句子，翻转句子中单词的顺序，但单词内字符的顺序不变。为简单起见，标点符号和普通字母一样处理。例如输入字符串"I am a student. "，则输出"student. a am I"。

方式一：

class Solution {
    public String reverseWords(String s) {
        //s="   the sky is blue  "
        //除去开头和结尾的空白字符s="the sky is blue"
        s = s.trim();
        // 正则匹配连续的空白字符作为分隔符分割
        //split返回的是String[]类型
        List<String> wordlist = Arrays.asList(s.split("\\s+"));
        // wordlist=["the","sky","is","blue"]
        Collections.reverse(wordlist);
        // wordlist=["blue","is","sky","the"]
        return String.join(" ",wordlist);

    }
}

方式二：先去掉所有多于的空格，然后将整个字符串反转，最后再将每个单词反转

class Solution {
    public String reverseWords(String s) {
       
        //去掉多于的空格
        StringBuilder sb = trimSpace(s);
        // 翻转字符串
        reverse(sb,0,sb.length()-1);
        // 翻转每个单词
        reverseEachWord(sb);

        return sb.toString();
    }

    public StringBuilder trimSpace(String s){
        int left = 0, right = s.length()-1;
        // 去掉字符串开头的空白字符
        while(left <= right && s.charAt(left) == ' '){
            left++;
        }
        // 去掉字符串末尾的空白字符
        while(left <= right && s.charAt(right) == ' '){
            right--;
        }
        // 将字符串间多余的空白字符去除
        StringBuilder sb = new StringBuilder();
        while(left <= right){
            char c = s.charAt(left++);
            if(c != ' '){
                sb.append(c);
            }else if(sb.charAt(sb.length()-1) != ' '){
                //如果s中连续2个空格，第一个空格放到sb中后，第二个空格就不用了
                sb.append(c);
            }
        }
        return sb;
    }

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

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

        while(left < n){
            // 循环至单词的末尾
            while(right < n && sb.charAt(right) != ' '){
                right++;
            }
            // 翻转单词
            reverse(sb,left,right-1);
            // 更新left，去找下一个单词
            left = right+1;
            right++;
        }
    }
}