[LeetCode 17] The letter combination of the phone number (medium) backtracking

Given a string containing only numbers 2-9, return all letter combinations it can represent.

The mapping of numbers to letters is given as follows (same as phone buttons). Note that 1 does not correspond to any letters.

Example:

Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Explanation:
Despite the above The answers are arranged in lexicographic order, but you can choose the order in which the answers are output.

Source: LeetCode (LeetCode)
link
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Code:

class Solution {
    
    
public:
    string t,s[8]={
    
    "abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};    
    //只有2-9按键有字母
    vector<string> ans;

    void dfs(string digits,int index)
    {
    
    
        if(index==digits.size()) //下标到了digits长度，算一个组合
        {
    
    
            ans.push_back(t); //加入ans数组
            return;
        } 
	
		//digits[index]-'0'-2 找到按键可以敲出的字符
        for(int i=0;i<s[digits[index]-'0'-2].size();i++) //
        {
    
    
            t+=s[digits[index]-'0'-2][i]; //累加字符
            dfs(digits,index+1); //下标加一
            t.erase(t.end()-1); //减去最后一个字符
        }
    }

    vector<string> letterCombinations(string digits) {
    
    
        if(!digits.size()) return ans; //空字符
        dfs(digits,0);
        return ans;
    }
};