leetcode第一刷_Letter Combinations of a Phone Number

map<char, string> num;

void allCombinations(string &digits, int start, int len, string &pres, vector<string> &res){
if(len == digits.length()){
res.push_back(pres);
//cout<<pres<<endl;
return;
}
for(int i=0;i<num[digits[start]].length();i++){
pres[len] = num[digits[start]][i];
allCombinations(digits, start+1, len+1, pres, res);
}
}

class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string> res;
if(digits.length()<=0){
res.push_back("");
return res;
}
num['2'] = "abc";
num['3'] = "def";
num['4'] = "ghi";
num['5'] = "jkl";
num['6'] = "mno";
num['7'] = "pqrs";
num['8'] = "tuv";
num['9'] = "wxyz";
string pres(digits.length(),'#');
allCombinations(digits, 0, 0, pres, res);
return res;
}
};