524. Longest Word in Dictionary through Deleting 

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class Solution {
    public String findLongestWord(String s, List<String> dictionary) {
        dictionary.sort((o1, o2) -> {
            int diff = Integer.compare(o2.length(), o1.length());
            if (diff != 0) {
                return diff;
            }
            return o1.compareTo(o2);
        });

        for (String word : dictionary) {
            if (match(word, s)) {
                return word;
            }
        }

        return "";
    }

    private boolean match(String word, String s) {
        if (word.length() > s.length()) {
            return false;
        }

        int i = 0, j = 0;
        while (i < word.length()) {
            while (j < s.length() && word.charAt(i) != s.charAt(j)) {
                j++;
            }

            // j == s.length() or word[i] == s[j]
            if (j == s.length()) {
                return false;
            } else {
                i++;
                j++; // 不要忘记匹配后移动 j 指针
            }
        }

        return true;
    }
}

References

524. Longest Word in Dictionary through Deleting