151. Reverse Words in a String

题目：

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

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

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

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.

链接： http://leetcode.com/problems/reverse-words-in-a-string/

6/13/2017

8ms, 64%

高亮部分是需要记住的Java String方法，reverse(char[], int start, int end)中start, end是左开右闭。

public class Solution {
    public String reverseWords(String s) {
        if (s == null || s.equals("")) {
            return s;
        }
        char[] characters = s.toCharArray();
        int validLength = 0;
        for (int i = 0; i < s.length(); i++) {
            if (characters[i] != ' ' || i != 0 && characters[i - 1] != ' ') {
                characters[validLength] = characters[i];
                validLength++;
            }
        }
        if (validLength == 0) {
            return "";
        }
        if (characters[validLength - 1] == ' ') {
            validLength -= 1;
        }

        char[] trimmedChars = new char[validLength];
        for (int i = 0; i < validLength; i++) {
            trimmedChars[i] = characters[i];
        }
        reverse(trimmedChars, 0, validLength);
        int startIndex = 0;
        for (int i = 1; i <= validLength; i++) {
            if (i == validLength || trimmedChars[i] == ' ') {
                reverse(trimmedChars, startIndex, i);
                startIndex = i + 1;
            }
        }
        return String.copyValueOf(trimmedChars);
    }
    private void reverse(char[] chars, int start, int end) {
        for (int i = start, j = end - 1; i <= j; i++, j--) {
            char tmp = chars[i];
            chars[i] = chars[j];
            chars[j] = tmp;
        }
        return;
    }
}

还可以用string直接来操作，留给二刷或者其他题目。

别人用trim(), split()的方法，原来多个whitespace是用split(" +")作为regex来表示的

https://discuss.leetcode.com/topic/11785/java-3-line-builtin-solution

public String reverseWords(String s) {
    String[] words = s.trim().split(" +");
    Collections.reverse(Arrays.asList(words));
    return String.join(" ", words);
}

更多讨论

https://discuss.leetcode.com/category/159/reverse-words-in-a-string