[LC] 151. Reverse Words in a String

Given an input string, reverse the string word by word.

Example 1:

Input: "the sky is blue"
Output: "blue is sky the"

Example 2:

Input: "  hello world!  "
Output: "world! hello"
Explanation: Your reversed string should not contain leading or trailing spaces.

class Solution {
    public String reverseWords(String s) {
        if (s == null || s.length() == 0) {
            return "";
        }
        char[] charArr = s.toCharArray();
        swap(charArr, 0, s.length() - 1);
        int i = 0, start = 0;
        while (i < s.length()) {
            if (i == 0 || charArr[i - 1] == ' ') {
                start = i;
            }
            
            if (i == charArr.length - 1 || charArr[i + 1] == ' ') {
                swap(charArr, start, i);
            }
            i += 1;
        }
        
        // need to trim space inside
        int slow = 0;
        for (int j = 0; j < charArr.length; j++) {
            if (charArr[j] == ' ' && (j == 0 || charArr[j - 1] == ' ')) {
                continue;
            }
            charArr[slow++] = charArr[j];
        }
        return new String(charArr, 0, slow).trim();
    }
    
    private void swap(char[] charArr, int i, int j) {
        while (i < j) {
            char tmp = charArr[i];
            charArr[i] = charArr[j];
            charArr[j] = tmp;
            i += 1;
            j -= 1;
        }
    }
}