# LeetCode 17. 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. Return the answer in **any order**.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

![](https://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/200px-Telephone-keypad2.svg.png)

**Example 1:**

```
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
```

**Example 2:**

```
Input: digits = ""
Output: []
```

**Example 3:**

```
Input: digits = "2"
Output: ["a","b","c"]
```

**Constraints:**

* `0 <= digits.length <= 4`
* `digits[i]` is a digit in the range `['2', '9']`.

## Solution:

[English Version in Youtube](https://youtu.be/czE2XcQjYyE)

[中文版解答Youtube Link](https://youtu.be/PvKXodU6irA)

[中文版解答Bilibili Link](https://www.bilibili.com/video/BV1tZ4y1A7Vg/)

```
class Solution {
    
public:
    vector<string> letterCombinations(string digits) {
        string symbols[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        vector<string> result;
        if (digits.empty()) {
            return result;
        }
        result.push_back("");
        
        for (char ch : digits) {
            int num = ch - '0';
            vector<string> new_result;
            for (const string& s : result) {
                for (char symbol : symbols[num]) {
                    string tmp = s + symbol;
                    new_result.push_back(tmp);
                }
            }
            result.swap(new_result);
        }
        
        return result;
    }
    
};
```
