class Solution {
    public String reverseWords(String s) {
        char[] chars = s.toCharArray();
        reverse(chars, 0, chars.length);
        int start = 0;
        for (int i = 0; i <= chars.length; i++) {
            // 字符串到头了，或者遇到了' '则进行翻转。注意！i == chars.length需要先判断！不让会数组越界异常！
            if (i == chars.length || chars[i] == ' ') {
                reverse(chars, start, i);
                start = i + 1;
            }
        }
        return new String(chars);
    }

    public void reverse(char[] chars, int start, int length) {
        for (int i = start, j = length - 1; i <= j; i++, j--) {
            char tmp = chars[i];
            chars[i] = chars[j];
            chars[j] = tmp;
        }
    }
}