【Leetcode】Letter Combinations of a Phone Number

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问题

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.

代码

void myletterCombinations(const char *s , vector<string> &v , string currentString){if (!s) {return;}if (*s != '\0') {char testA[12];char a = ('a' + ((*s-'2') * 3));//sprintf(testA, "%c" , a + 4);/* get next */switch (*s) {case '9':myletterCombinations(s + 1, v, currentString + string("w"));myletterCombinations(s + 1, v, currentString + string("x"));myletterCombinations(s + 1, v, currentString + string("y"));myletterCombinations(s + 1, v, currentString + string("z"));break;case '8':myletterCombinations(s + 1, v, currentString + string("t"));myletterCombinations(s + 1, v, currentString + string("u"));myletterCombinations(s + 1, v, currentString + string("v"));break;case '7':myletterCombinations(s + 1, v, currentString + string("p"));myletterCombinations(s + 1, v, currentString + string("q"));myletterCombinations(s + 1, v, currentString + string("r"));myletterCombinations(s + 1, v, currentString + string("s"));break;case '6':case '5':case '4':case '3':case '2':{sprintf(testA, "%c" , a);myletterCombinations(s + 1, v, currentString + string(testA));sprintf(testA, "%c" , a+1);myletterCombinations(s + 1, v, currentString + string(testA));sprintf(testA, "%c" , a +2);myletterCombinations(s + 1, v, currentString + string(testA));break;}default:myletterCombinations(s + 1, v, currentString);break;}}else{v.push_back(currentString);}}class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> ss;myletterCombinations(digits.c_str(), ss, string(""));return ss;    }};

分析

这里注意不仅仅是 9 有四个字符，7也是，我开始没注意。 2-6 是没问题的，使用通用的算法计算就可以了。 7,9 不行，8自然在中间也不能独自计算。

递归之就行了。跳过了 0 , 1, #,* 等无意义的输入，我没有试过假设不处理这几个字符是否会拒绝我的代码。

总结

递归各种组合。递归之前想好递归的停止条件是啥。

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