```
class Solution {
public:
string countAndSay(int n) {
string s="1";
for(int i=1; i<n; ++i){
string t;
for(int j=0; j<s.length();){
char c=s[j];
int cnt=0;
while(j<s.length() && c==s[j]){
++j; ++cnt;
}
t.push_back('0'+cnt);
t.push_back(c);
}
swap(s, t);
}
return s;
}};
```

My question is, it looks that the string sequence would only have '1', '2', '3'. Can we prove this?