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.
用dfs解决
class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string>ans;
dfs(digits,"",ans);
return ans;
}
void dfs(string digits,int start,string s,vector<string>&ans){
if(start == digits.size()){
if(s != "")
ans.push_back(s); return;
}
int cnt = digits[start]-'2';
for(int i = 0; i < 4; i++)
if(letter[cnt][i] != '!')
dfs(digits,start+1,s+letter[cnt][i],ans);
}
char letter[8][4] = {
{'a','b','c','!'},{'d','e','f',{'g','h','i',{'j','k','l',{'m','n','o',{'p','q','r','s'},{'t','u','v',{'w','x','y','z'}
};
};
