leetcode Letter Combinations of a Phone Number

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

Description

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.

dfs。。

class Solution {
public:
vector<string> letterCombinations(string digits) {
init();
ans.clear();
n = digits.length();
if (!n) return ans;
dfs(0, "", digits);
return ans;
}
private:
size_t n;
map<int, string> A;
vector<string>ans;
void init() {
A[2] = "abc";
A[3] = "def";
A[4] = "ghi";
A[5] = "jkl";
A[6] = "mno";
A[7] = "pqrs";
A[8] = "tuv";
A[9] = "wxyz";
}
void dfs(int cur, string x, string digits) {
if (cur == n) { ans.push_back(x); return; }
int d = digits[cur] - '0';
for (size_t i = 0; i < A[d].size(); i++) {
dfs(cur + 1, x + A[d][i], digits);
}
}
};