17.Letter Combinations of a Phone Number

Tags: Medium String Backtracking

Links: https://leetcode.com/problems/letter-combinations-of-a-phone-number/

---

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "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 {
    map<char, string> m = {{'2',"abc"}, {'3', "def"}, {'4', "ghi"}, {'5', "jkl"}, {'6', "mno"},{'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"}};

public:
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if (digits.empty()) return res;

        if (digits.size() == 1) {
            string tmp = m[digits[0]];
            for (auto e : tmp) {
                string str;
                str.push_back(e);
                res.push_back(str);
            }
        }
        else {
            vector<string> store = letterCombinations(digits.substr(1));
            string tmp = m[digits[0]];
            for (int i = 0; i < tmp.size(); ++i) {
                string str; str.push_back(tmp[i]);
                for (int j = 0; j < store.size(); ++j) {
                    res.push_back(str + store[j]);
                }
            }
        }

        return res;
    }
};

这道题目其实考察的就是简单的回溯法，然而很神奇的是它的时间判定：

Runtime: 8 ms, faster than 5.54% of C++ online submissions for Letter Combinations of a Phone Number.
Memory Usage: 8.6 MB, less than 82.86% of C++ online submissions for Letter Combinations of a Phone Number.

当然也可以把字典换成vector，效率并没有太多的提升。另外需要注意输入字符串为空的情况。