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".

Solution:

public class Solution {
  public String reverseWords(String s) {
    if (s == null) return null;
    char[] a = s.toCharArray();
    int n = a.length;
    // step 1. reverse the whole string
    reverse(a, 0, n - 1);
    // step 2. reverse each word
    reverseWords(a, n);
    // step 3. clean multiple spaces
    return cleanSpaces(a, n);
  }
  void reverseWords(char[] a, int n) {
    int i = 0, j = 0;
    while (i < n) {
      while (i < j || i < n && a[i] == ' ') i++; // skip spaces
      while (j < i || j < n && a[j] != ' ') j++; // skip non spaces
      reverse(a, i, j - 1);                      // reverse the word
    }
  }
  // trim leading, trailing and multiple spaces
  String cleanSpaces(char[] a, int n) {
    int i = 0, j = 0;
    while (j < n) {
      while (j < n && a[j] == ' ') j++;             // skip spaces
      while (j < n && a[j] != ' ') a[i++] = a[j++]; // keep non spaces
      while (j < n && a[j] == ' ') j++;             // skip spaces
      if (j < n) a[i++] = ' ';                      // keep only one space
    }
    return new String(a).substring(0, i);
  }
  // reverse a[] from a[i] to a[j]
  private void reverse(char[] a, int i, int j) {
    while (i < j) {
      char t = a[i];
      a[i++] = a[j];
      a[j--] = t;
    }
  }
}