解题思路：
题目本身挺简单的，考虑用set，判断每个单词的字母是不是属于同一个集合。需要注意的是：1）set的构造方法；2）单词可能是大小写混合的，不一定只是首字母大写；
3）break是跳出循环=。=switch写的时候要注意，不好用的话还是用if-else好了。
vector<string> findWords(vector<string>& words) {
char f[20] = {'q','w','e','r','t','y','u','i','o','p','Q','W','E','R','T','Y','U','I','O','P'};
set<char>first(f,f+20);
char s[18] = {'a','s','d','f','g','h','j','k','l','A','S','D','F','G','H','J','K','L'};
set<char>second(s, s+18);
char t[14] = {'z','x','c','v','b','n','m','Z','X','C','V','B','N','M'};
set<char>third(t, t+14);
vector<string>::iterator it;
string temp = "";
vector<string> result;
bool flag = true;
int i;
for (it = words.begin(); it != words.end(); it++) {
temp = *it;
flag = true;
if (temp.length() == 1) {
result.insert(result.end(), temp);
continue;
}
if (first.find(temp[0]) != first.end()) {
for (i = 1; i < temp.length(); i++) {
if (first.find(temp[i]) == first.end()) {
break;
}
}
if (i != temp.length()) {
flag = false;
continue;
}
}
else if (second.find(temp[0]) != second.end()) {
for (i = 1; i < temp.length(); i++) {
if (second.find(temp[i]) == second.end()) {
break;
}
}
if (i != temp.length()) {
flag = false;
continue;
}
}
else {
for (i = 1; i < temp.length(); i++) {
if (third.find(temp[i]) == third.end()) {
break;
}
}
if (i != temp.length()) {
flag = false;
continue;
}
}
if (flag == true) {
result.insert(result.end(), temp);
}
}
return result;
}