Restore IP Addresses

 1 class Solution {
 2 public:
 3     vector<string> restoreIpAddresses( string s ) {
 4         string ip;
 5         ProcessSub( ip, 0, s );
 6         return address;
 7     }
 8 private:
 9     void ProcessSub( string ip, int step, string str ) {
10         if( step == 4 ) { address.push_back( ip ); return; }
11         int sLen = str.length();
12         if( sLen < 4-step || sLen > 3*(4-step) ) { return; }
13         for( int i = max( 1, sLen-3*(3-step) ); i <= min( 3, sLen-(3-step) ); ++i ) {
14             string subStr = str.substr( 0, i );
15             if( IsLegal( subStr ) ) {
16                 if( !ip.empty() ) { subStr = ip + "." + subStr; }
17                 ProcessSub( subStr, step+1, str.substr( i ) );
18             }
19         }
20     }
21     bool IsLegal( const string& str ) {
22         if( str.length() == 1 ) { return true; }
23         if( str[0] == ‘0‘ ) { return false; }
24         int value = 0;
25         for( size_t i = 0; i != str.length(); ++i ) {
26             value = 10 * value + str[i] - ‘0‘;
27         }
28         if( value > 255 ) {  return false; }
29         return true;
30     }
31     vector<string> address;
32 };