class Solution {
    public void reverseStr(char[] s, int l, int r) {
        while(l < r) {
            char temp = s[l];
            s[l++] = s[r];
            s[r--] = temp;
        }
    }

    public String reverseWords(String s) {
        char[] str = s.toCharArray();
        int n = str.length;

        reverseStr(str, 0 , n - 1);
        
        int slow = 0;
        int fast = 0;

        while(fast < n) {
            // skip whitespace
            while(fast < n && str[fast] == ' ') fast++;
            // add whitespace
            if(slow > 0 && fast < n) str[slow++] = ' ';
            int start = slow;
            // move word to correct place
            while(fast < n && str[fast] != ' ') str[slow++] = str[fast++];
            // reverse each word
            reverseStr(str, start, slow-1);
        }

        // strim useless space
        return new String(str).substring(0, slow);
    }
}