[Leetcode] 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.

public class Solution {
    public String reverseWords(String s) {
        if(s == null)
            return null;
        if(s.length() == 0)
            return s;
        char[] array = s.toCharArray();
        reverse(array, 0, array.length - 1);
        int index = -1;
        int start = 0;
        int space = -1;
        while(start < array.length && array[start] == ' ')
            start++;
        while(start < array.length){
            if(index == -1 || array[start] != ' ')
                array[++index] = array[start++];
            else if(array[start] == ' ' && array[index] == ' '){
                start++;
            }
            else {// array[start] == ' ' && array[index] != ' ' 
                reverse(array, space + 1, index);
                array[++index] = array[start++];
                space = index;
            }
        }
        reverse(array, space + 1, index);
        if(index == -1)
            return new String("");
        else{
            if(index < array.length && array[index] == ' ')
                return new String(array, 0, index);
            else
                return new String(array, 0, index + 1);
        }
    }
    public void reverse(char[] array, int left, int right){
        while(left < right){
            char temp = array[left];
            array[left] = array[right];
            array[right] = temp;
            left++;
            right--;
        }
    }
}

Use Java String.split()

public class Solution {
    public String reverseWords(String s) {
        if(s == null)
            return null;
        if(s.length() == 0)
            return s;
        String[] res = s.split(" ");
        StringBuilder sb = new StringBuilder("");
        for(int i = res.length - 1; i >= 0; i--)
            if(res[i].length() != 0){
                sb.append(' ');
                sb.append(res[i]);
            }
        if(sb.length() == 0)
            return sb.toString();
        return sb.substring(1, sb.length());
    }
}