LeetCode17：电话号码的字母组合

 1 class Solution {
 2 public:
 3     vector<string> ans;
 4     string gen;
 5     string dict[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
 6     vector<string> letterCombinations(string digits) {
 7         if(digits.empty()) return ans;
 8         gen.resize(digits.size());
 9         backtrace(digits,0);
10         return ans;
11     }
12 
13     void backtrace(string &digits, int n){
14         if(n==digits.size()){
15             ans.push_back(gen);
16             return;
17         }
18         int w=digits[n]-‘0‘;
19         for(int i=0;i<dict[w].size();i++){
20             gen[n]=dict[w][i];
21             backtrace(digits,n+1);
22         }
23 
24     }
25 };