class Solution {
    public String reverseWords(String s) {
        char[] ca = s.toCharArray();
        int len = ca.length;

        reverse(ca, 0, len - 1);

        int r, l = 0;
        while (l < len) {
            while (l < len && ca[l] == ' ') l++;
            r = l;
            while (r < len && ca[r] != ' ') r++;

            reverse(ca, l, r == len-1 ? len-1: r-1);
            l = r;
        }

        return new String(ca);
    }

    private void reverse(char[] ca, int l, int r) {
        while (l < r) {
            char t = ca[l];
            ca[l] = ca[r];
            ca[r] = t;
            l++;
            r--;
        }
    }
}