Leetcode题解-17. Letter Combinations of a Phone Number

Leetcode题解-17. Letter Combinations of a Phone Number

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.

思路

这道题思维比较简单，就是不断扩展字符串的长度，每次扩展形成的字符串是原来的三或四倍。

代码

vector<string> letterCombinations(string digits) {        if(digits.empty()) return vector<string>();        char letters[8][4] = {{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};        int l = digits.size();        vector<string> res;        res.push_back("");        for(int i = 0; i < l; i++){            vector<string> tem;            int count;            if(digits[i] == '7' || digits[i] == '9') count = 4;            else if(digits[i] > '1' && digits[i] < '9') count = 3;            else break;            for(int j = 0; j < count; j++){                vector<string>::iterator it;                for(it = res.begin(); it < res.end(); it++){                    tem.push_back((*it) + letters[digits[i] - '2'][j]);                }            }            res.swap(tem);        }        return res;    }