【LeetCode在线编程记录-1】字符串按单词反转

写在前面

   LeetCode(地址：https://oj.leetcode.com/)是一个在线编程网站，题目经典，测试用例完备，共计157道算法类的题目。之后我会记录我的一些练习题目，有些答案是我自己原创的（说是原创，也很可能是之前在别的地方看到的而已），有些是从讨论区看到的，我都会明确标注出处。

Reverse Words in a String

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.

【我的分析】原来希望通过字符串全部反转然后按单词进行反转即可，但是遇到的困难是多个空格如何处理的问题。

【分析1-非原创】需要考虑的情况是单词与单词之间可能是多个空格。从前往后扫描得到字符temp，最后返回的是字符串result：

1）如果不是空格，word+=temp；

2）如果是空格，将word附加到result前面。

参考：https://oj.leetcode.com/discuss/10888/my-java-solution-few-lines

   

 /* 方法1：二级缓存的思想，word来存储字符，遇到空格,如果word不是空格，将word添加到字符串的newStr前面，同时置为"" */    public static String reverseWords4(String s) {        String temp = "";        String result = "";        for (int i = 0; i < s.length(); i++) {            char c = s.charAt(i);            if (c == ' ') {                if (temp != "" && result != "") {                    result = temp + " " + result;                }                if (temp != "" && result == "") {                    result = temp;                }                temp = "";             } else {                temp += c;            }        }        /* 最后一次添加 */        if (temp != "" && result != "") {            result = temp + " " + result;        }        if (temp != "" && result == "") {            result = temp;        }        return result;    }

【分析2-非原创】利用正则表达式将字符串分隔成字符串数组。这要用到系统函数。

参考：https://oj.leetcode.com/discuss/9142/my-accepted-java-solution

方案一：系统函数trim()和split()

/* 正则表达\s+表示任意多个空白字符 */    public static String reverseWords3(String s) {        String[] parts = s.trim().split("\\s+");        String out = "";        if (parts.length > 0) {            for (int i = parts.length - 1; i > 0; i--) {                out += parts[i] + " ";            }            out += parts[0];        }        return out;    }

方案二：系统类Scanner默认构造函数会将字符串按空白字符分隔

   

 public String reverseWords(String s) {        Scanner parts = new Scanner(s);         String result = "";         while(parts.hasNext()){            result = parts.next() + " " + result;        }         return result.trim();    }