剑指 Offer 58 - I. 翻转单词顺序

class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();
        int j = s.length() - 1;

        while (j >= 0) {
            // skip whitespace
            while (j >= 0 && s.charAt(j) == ' ') {
                j--;
            }

            // j points to word or out of index
            if (j == -1) {
                break; // 注意此处跳出而不是返回空字符串，因为 sb 中可能有字符
            }

            int i = j;
            while (i >= 0 && s.charAt(i) != ' ') {
                i--;
            }
            
            // i points to whitespace or out of index
            sb.append(s, i + 1, j + 1).append(' ');
            j = i - 1;
        }

        if (sb.length() == 0) {
            return "";
        } else {
            return sb.deleteCharAt(sb.length() - 1).toString();
        } 
    }
}

References

剑指 Offer 58 - I. 翻转单词顺序