[leetCode 解题报告]017. Letter Combinations of a Phone Number

Description：

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.

解题思路：

题意给定一个数字字符串，返回可能由手机九空格输入法上字母的所有组合情况，顺序没有要求。

算法思路：

• 步骤1 如果数字字符串digits为空，则返回也为空；
• 步骤2 将0-9数字字符与手机上的九宫格输入法上的字符对应起来，即vector<string> s的建立映射关系；
• 步骤3 res保存上一个数字的最终结果，tmp保存当前数字的映射结果，将res中所有字符串与num中的所有字符逐一拼接，然后保存到tmp中，如此迭代，直到所有digits中的数字字符处理结束，详细见代码，好难表达；

代码如下：

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        
        vector<string> res;
        
        if(digits.length() == 0)
            return res;
        
        res.push_back("");
        vector<string> s = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        
        for(int i = 0; i < digits.size(); i ++) {
            
            string num = s[digits[i] - '0'];
            vector<string> tmp;
            for(int j = 0; j < num.length(); j ++) {
                for(int k = 0; k < res.size(); k ++)
                    tmp.push_back(res[k] + num[j]);
            }
            res = tmp;
        }
        return res;
    }
};