比较字符的大小时，若字符串a的首字符大于b的首字符则a比较大，不对后续字符进行判断。
class Solution {
public:
string findLongestWord(string s, vector<string>& d) {
string ans;
for (int i = 0; i < d.size(); i++) {
int pi = 0, pj = 0;
for (; pi < s.size() && pj < d[i].size(); pi++) {
pj += s[pi] == d[i][pj];
}
if (pj == d[i].size() && (ans.size() < d[i].size() || (ans.size() == d[i].size() && ans > d[i])))
ans = d[i];
}
return ans;
}
};
