思路：找出s中包含的最长dictionary
外层遍历dictionary，内层双指针
如果遇到一个匹配的dictionary，就先比较长度，再比较字典序

class Solution:
    def findLongestWord(self, s: str, dictionary: List[str]) -> str:
        res=""
        for d in dictionary:
            i=j=0
            while i<len(d) and j<len(s):
                if d[i]==s[j]:
                    i+=1
                j+=1
            if i==len(d):
                if len(d)>len(res) or (len(d)==len(res) and d<res):
                    res=d
        return res

class Solution {
    public String findLongestWord(String s, List<String> dictionary) {
        String res="";
        for(String d : dictionary){
            int i=0,j=0;
            while(i<s.length() && j<d.length()){
                if(s.charAt(i)==d.charAt(j)){
                    j++;
                }
                i++;
            }
            if(j==d.length()){
                if(d.length()>res.length() || 
                (d.length()==res.length() && d.compareTo(res)<0)){
                    res=d;
                }
            }        
        }
        return res;
    }
}

也可以先将dictionary按长度和字典序排好，再遍历双指针

class Solution {
    public String findLongestWord(String s, List<String> dictionary) {
        Collections.sort(dictionary, (a,b)->{
            if (a.length() != b.length()) return b.length() - a.length();
            return a.compareTo(b);
        });
        int n = s.length();
        for (String d : dictionary) {
            int m = d.length();
            int i = 0, j = 0;
            while (i < n && j < m) {
                if (s.charAt(i) == d.charAt(j)) j++;
                i++;
            }
            if (j == m) return d;
        }
        return "";
    }
}