leetcode | Letter Combinations of a Phone Number

问题

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.

解析

//时间复杂度 O(3^n)， 空间复杂度O(n) ； n为digit的位数
const string key[] = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string> result;
if (digits.size() == 0)
return result;
string str(digits.size(), ' ');  //为单个字符串分配空间
PushString(digits, 0, str, result);
return result;
}

private:
void PushString(string& digits, int k, string &str, vector<string>& result) {
if (k == digits.size()) {
result.push_back(str);
return;
}
for (int i = 0; i < key[digits[k] - '0'].size(); i++) {
str[k] = key[digits[k] - '0'][i];  //因为每一个str的长度都等于digits的大小，所以str可以重用（完全覆盖）
PushString(digits, k + 1, str, result);
}

}
};