leetcode 17 Letter Combinations of a Phone Number

问题

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

解法

回溯法， 递归

class Solution {public:    void lc(vector<string>& ret, char* s, int sCnt, const char* digits, int depth)    {        if (digits[depth] == 0)        {            if (sCnt== 0)                return;            s[sCnt] = 0;            string now(s);            ret.push_back(now);            return;        }        if (digits[depth] < '2')        {            lc(ret, s, sCnt, digits, depth+1);            return;        }        char start, end;        switch(digits[depth])        {            case '7':                start = 'p';                end = 't';                break;            case '8':                start = 't';                end = 'w';                break;            case '9':                start = 'w';                end = 'z'+1;                break;            default:                start = (digits[depth]-'2') * 3+ 'a';                end = start +3;                break;        }        for (char c = start; c<end; c++)        {            s[sCnt] = c;            lc(ret, s, sCnt+1, digits, depth+1);        }    }    vector<string> letterCombinations(string digits) {        char s[digits.size()+1];        vector<string> ret;        lc(ret, s, 0, digits.c_str(), 0);        return ret;    }};