151. Reverse Words in a String

class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();

        for (int j = s.length() - 1; j >= 0; j--) {
            if (s.charAt(j) != ' ') {
                // 找到单词右边界
                if (sb.length() != 0) {
                    sb.append(" ");
                }

                int i = j; // word: [i, j]
                while (i >= 0 && s.charAt(i) != ' ') {
                    i--;
                }

                // now: i == -1 or s.charAt(i) == ' '
                sb.append(s, i + 1, j + 1);
                j = i;
            }
        }

        return sb.toString();
    }
}

Reference

151. Reverse Words in a String
剑指 Offer 58 - I. 翻转单词顺序