public String reverseWords(String s) {

if (s == null) return s;

String[] words = s.replaceAll("^\\s+", "").split("\\s+", 0);

int i = 0;

while (i < words.length/2) {

swap(words, i, words.length-i-1);

i++;

}

return String.join(" ", words);

}

private void swap(String[] words, int i, int j) {

String temp = words[i];

words[i] = words[j];

words[j] = temp;

}

/**

public String reverseWords2(String s) {

String[] words = s.trim().split(" +");

Collections.reverse(Arrays.asList(words));

return String.join(" ", words);

}

public String reverseWords3(String s) {

if (s == null || s.length() == 0){

return s;

}

String[] array = s.split(" ");

StringBuilder res = new StringBuilder();

for (int i = array.length - 1; i >= 0; i--){

if(array[i].length() != 0){

if (res.length() > 0){

res.append(" ");

}

res.append(array[i]);

}

}

return res.toString();

}

/**

public String reverseWords4(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 up 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;

}

}

public String reverseWords5(String s) {

if (s == null || s.length() == 0) return s;

char[] chars = s.trim().toCharArray();

int N = chars.length;

char[] res = new char[N];

int right = N - 1;

int left = 0;

while (right >= 0) {

while (right >= 0 && chars[right] == ' ') {

right--;

}

if (right < 0) break;

int end = right;

while (right >= 0 && chars[right] != ' ') {

right--;

}

int i = right + 1;

while (i <= end) {

res[left++] = chars[i++];

}

if (right < 0) break;

res[left++] = ' ';

}

return new String(res, 0, left);

}