Reverse Words in a String I & Reverse Words in a String II

Reverse Words in a String I

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

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

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.

分析：

先取出多余的white space, 然后再进行(A^TB^T)^T = BA的转换。

 public class Solution {
     public String reverseWords(String s) {
         if (s == null || s.length() < ) return s;
         int i = ;
         StringBuilder sb = new StringBuilder();
         while (i < s.length()) {
             StringBuilder temp = new StringBuilder();
             while(i < s.length() && !(s.charAt(i) == ' ')) {
                 temp.insert(, s.charAt(i));
                 i++;
             }
             if (temp.length() != ) {
                 sb.append(temp.toString());
                 sb.append(' ');
             }
             while(i < s.length() && s.charAt(i) == ' ') {
                 i++;
             }
         }
         if (sb.length() > ) {
             sb.deleteCharAt(sb.length() - );
         }
         char[] arr = sb.toString().toCharArray();
         reverse(arr);

         return new String(arr);
     }

     public void reverse(char[] arr) {
         int i = , j = arr.length - ;
         while (i < j) {
             char temp = arr[i];
             arr[i] = arr[j];
             arr[j] = temp;
             i++;
             j--;
         }
     }
 }

 Reverse Words in a String II

Given a string, you need to reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.

Example 1:

Input: "Let's take LeetCode contest"
Output: "s'teL ekat edoCteeL tsetnoc"

Note: In the string, each word is separated by single space and there will not be any extra space in the string.

 public class Solution {
     public String reverseWords(String s) {
         char[] ca = s.toCharArray();
         int start = ;
         for (int i = ; i <= ca.length; i++) {
             if (i == ca.length || ca[i] == ' ') {
                 reverse(ca, start, i - );
                 start = i + ;
             }
         }
         return new String(ca);
     }

     private void reverse(char[] ca, int i, int j) {
         for (; i < j; i++, j--) {
             char tmp = ca[i];
             ca[i] = ca[j];
             ca[j] = tmp;
         }
     }
 }