LeetCode—17.Letter Combinations of a Phone Number

Type：medium

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "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.

---

本题题意为给定一串数字，按照9宫格对应关系，输出所有可能代表的字符串。

首先建立v字符数组，建立每个数字与对应的所有字符的关系。建立返回的ret这个vector，赋一个空的初值，否则接下来的程序处理会报错。

挨个读取digits字符串的数字，定位这个数字所代表的字符串，将原ret容器中所有字符串加上这个数字所代表的的所有字符即可。

如代码中所示，temp这个vector进行了时间复杂度上为n^2次操作，负责将原ret容器的每个字符串首先加上vv这个字符串中第一个字符，再将ret原字符串加上vv第二个字符，直至加到数字num对应的容器v中的字符串（vv）最后一位。最后将这个temp赋给ret。

---

class Solution {

public:

    vector letterCombinations(string digits) {

        int n = digits.length();

        if(n == 0) return vector();

        vector ret;

        vector v = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};

        ret.push_back("");  //initialize

        vector temp;

        for(int i=0; i

            int num = digits[i] - '0';

            string vv = v[num];

            int len = vv.length();

            for(int j=0; j

                for(int m=0; m

                    temp.push_back(ret[j] + vv[m]);

                }

            }

            ret.swap(temp);

            temp.clear();

        } 

        //ret.erase(ret.begin());

        return ret;

    }

};