151. 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() < 1) return s;
        int i = 0;
        StringBuilder sb = new StringBuilder();
        while (i < s.length()) {
            StringBuilder temp = new StringBuilder();
            while(i < s.length() && !(s.charAt(i) == ' ')) {
                temp.insert(0, s.charAt(i));
                i++;
            }
            if (temp.length() != 0) {
                sb.append(temp.toString());
                sb.append(' ');
            }
            while(i < s.length() && s.charAt(i) == ' ') {
                i++;
            }
        }
        if (sb.length() > 0) {
            sb.deleteCharAt(sb.length() - 1);
        }
        char[] arr = sb.toString().toCharArray();
        reverse(arr);
        
        return new String(arr);
    }

    public void reverse(char[] arr) {
        int i = 0, j = arr.length - 1;
        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 = 0;
        for (int i = 0; i <= ca.length; i++) {
            if (i == ca.length || ca[i] == ' ') {
                reverse(ca, start, i - 1);
                start = i + 1;
            }
        }
        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;
        }
    }
}