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# 27 151

First, trim the leading and trailing spaces and extra spaces in the word. Then reverse the whole string. Then reverse each word in the string. 

Time O(N)

Space O(N)

class Solution { public String reverseWords(String s) { // converst string to string builder // and trim spaces at the same time StringBuilder sb = trimSpaces(s); // reverse the whole string reverse(sb, 0, sb.length() - 1); // reverse each word reverseEachWord(sb);

```
return sb.toString();
```

}

```
public StringBuilder trimSpaces(String s) {
int left = 0, right = s.length() - 1;
// remove leading spaces
while (left <= right && s.charAt(left) == ' ') ++left;

// remove trailing spaces
while (left <= right && s.charAt(right) == ' ') --right;

// reduce multiple spaces to single one
StringBuilder sb = new StringBuilder();
while (left <= right) {
  char c = s.charAt(left);

  if (c != ' ') sb.append(c);
  else if (sb.charAt(sb.length() - 1) != ' ') sb.append(c);

  ++left;
}
return sb;
```

}

public void reverse(StringBuilder sb, int left, int right) { while (left < right) { char tmp = sb.charAt(left); sb.setCharAt(left++, sb.charAt(right)); sb.setCharAt(right--, tmp); } }

public void reverseEachWord(StringBuilder sb) { int n = sb.length(); int start = 0, end = 0;

```
while (start < n) {
  // go to the end of the word
  while (end < n && sb.charAt(end) != ' ') ++end;
  // reverse the word
  reverse(sb, start, end - 1);
  // move to the next word
  start = end + 1;
  ++end;
}
```

}

}