leetcode——17——Letter Combinations of a Phone Number

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.

class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string> ret;
if(digits == "")
return ret;

ret.push_back("");

vector<string> dict(10); //0~9
dict[2] = "abc";
dict[3] = "def";
dict[4] = "ghi";
dict[5] = "jkl";
dict[6] = "mno";
dict[7] = "pqrs";
dict[8] = "tuv";
dict[9] = "wxyz";

for(int i = 0; i < digits.size(); i ++)
{
int size = ret.size();
for(int j = 0; j < size; j ++)
{
string cur = ret[0];
ret.erase(ret.begin());
for(int k = 0; k < dict[digits[i]-'0'].size(); k ++)
{
ret.push_back(cur + dict[digits[i]-'0'][k]);
}
}
}
return ret;

}
};