Letter Combinations of a Phone Number

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

 1 class Solution {
 2 public:
 3     vector<string> letterCombinations( string digits ) {
 4         vector<string> combinations[2] = { vector<string>( 1, "" ) };
 5         for( size_t i = 0; i != digits.length(); ++i ) {
 6             combinations[(i+1)%2].clear();
 7             for( auto iter = combinations[i%2].begin(); iter != combinations[i%2].end(); ++iter ) {
 8                 if( digits[i] >= ‘2‘ && digits[i] <= ‘8‘ && digits[i] != ‘7‘ ) {
 9                     for( int k = 0; k < 3; ++k ) {
10                         combinations[(i+1)%2].push_back( *iter );
11                         combinations[(i+1)%2].back().push_back( digits[i] != ‘8‘ ? 3*(digits[i]-‘2‘)+k+‘a‘ : k+‘t‘ );
12                     }
13                 } else if( digits[i] == ‘7‘ || digits[i] == ‘9‘ ) {
14                     for( int k = 0; k < 4; ++k ) {
15                         combinations[(i+1)%2].push_back( *iter );
16                         combinations[(i+1)%2].back().push_back( digits[i] == ‘7‘ ? k+‘p‘ : k+‘w‘ );
17                     }
18                 }
19             }
20         }
21         return combinations[digits.length()%2];
22     }
23 };