题目描述：
Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.
Input:Digit string "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.
这道题不算难，想想就出来了。
solution：
vector<string> letterCombinations(string digits) { int n = digits.size(); vector<string> res; if(n == 0) return res; res.push_back(""); string numap[] = {"#","#","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"}; for (int i = 0;i < n;++i) { vector<string> tmp; for(int j = 0;j < res.size();++j) for(int k = 0;k < numap[digits[i]-‘0‘].size();++k) tmp.push_back(res[j] + numap[digits[i]-‘0‘][k]); res = tmp; } return res; }
此题还有其他解法，《编程之美》中也有相似题目，暂时先这样了。
Letter Combinations of a Phone Number
原文：http://www.cnblogs.com/gattaca/p/4315884.html