1 #include <vector>
 2 #include <iostream>
 3 #include <string>
 4 #include <assert.h>
 5 
 6 using namespace std;
 7 
 8 class Solution {
 9     
10 private:
11     const string letterMap[10] = {
12         " ",
13         "",
14         "abc",
15         "def",
16         "ghi",
17         "jkl",
18         "mno",
19         "pqrs",
20         "tuv",
21         "wxyz"
22     };
23     
24     vector<string> res;
25     // 处理第index位数字 
26     // s保存digits[0...index-1]生成的字符串 
27     // 找到和digits[index]匹配的字母，获得digits[0...index]生成的解 
28     void findCombination(const string &digits, int index, const string &s){
29         
30         cout<<index<<" : "<<s<<endl;
31         
32         // 终止条件 
33         if( index == digits.size() ){
34             // s是一个解，保存 
35             res.push_back(s);
36             cout<<"get "<<s<<" , return"<<endl;
37             return;
38         }
39         
40         char c = digits[index];
41         assert( c >= '0' && c <= '9' && c != '1' );
42         string letters = letterMap[c-'0']; 
43          
44         for( int i = 0 ; i < letters.size() ; i ++ ){
45             cout<<"digits["<<index<<"] = "<<c<<" , use "<<letters[i]<<endl;
46             // 处理第index+1位数字
47             findCombination(digits, index + 1, s + letters[i] );
48         }
49         cout<<"digits["<<index<<"] = "<<c<<" complete, return"<<endl;
50         return;
51     }
52 public:
53     vector<string> letterCombinations(string digits) {
54         
55         // 初始化 
56         res.clear();
57         // 边界情况 
58         if( digits == "" )
59             return res;
60         
61         findCombination(digits, 0, "");
62         
63         return res;
64     }
65 };
66 
67 int main(){
68     
69     vector<string> res = Solution().letterCombinations("23");
70     for( int i = 0 ; i < res.size() ; i ++ )
71         cout<<res[i]<<endl;
72         
73     return 0;
74 }