19.1.26 [LeetCode17] Letter Combinations of a Phone Number

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.

题意

字母和数字间存在映射，给出一串数字求可以映射到的所有字母串

感觉好像没啥意思

题解

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> Old, New;
        Old.push_back("");
        int f[10][2] = { -1,-1,-1,-1,0,2,3,5,6,8,9,11,12,14,
        15,18,19,21,22,25 };
        for (int i = 0; digits[i]; i++) {
            New.clear();
            int idx = digits[i] - '0';
            for (int j = 0; j < Old.size(); j++) {
                for (int k = f[idx][0]; k <= f[idx][1]; k++)
                    New.push_back(Old[j] + (char)(k + 'a'));
            }
            Old = New;
        }
        return New;
    }
};