[leetcode刷题系列]Restore IP Addresses

class Solution {
bool isValid(string s){
if(s.size() == 1)
return true;
if(s[0] == '0' || s.size() > 3)
return false;
int ret = 0;
for(int i = 0; i < s.size(); ++ i)
ret = ret * 10 + s[i] - '0';
return ret <= 255;

}
vector<string> dfs(string s, int num){
vector<string> vs;
if(num == 1){
if(isValid(s))
vs.push_back(s);
return vs;
}
for(int i = 0; i < 3 && i + 1 < s.size(); ++ i){
string now = s.substr(0, i + 1);
if(isValid(now)){
vector<string> ret = dfs(s.substr(i + 1, s.size() - i - 1), num - 1);
for(int j = 0; j < ret.size(); ++ j)
vs.push_back(now + '.' + ret[j]);
}
}
return vs;
}
public:
vector<string> restoreIpAddresses(string s) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
return dfs(s, 4);
}
};