Leetcode 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.

本题利用队列实现即可

如“23”，

首先获取2

然后相对列中插入a,b,c

然后弹出a，在a后面附件下面一个数字，即ad,ae,af,插入队列即可

vector<string> letterCombinations(string digits) {
    vector<string> res;
    if(digits.length() == 0) {res.push_back("");return res;}
    string num_map[10]={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    queue<string> que;
    int n = num_map[digits[0]-'0'].length();
    if(n == 0) que.push("");
    else{
        for(int i = 0; i <n;  ++ i){
            string tmp = string(1,num_map[digits[0]-'0'][i]);
            que.push(tmp);
        }
    }
    int index =1;
    while(!que.empty() && index < digits.length()){
        int cnt = que.size();
        while(cnt-->0){
            string a = que.front();que.pop();
            int len = num_map[digits[index]-'0'].length();
            if(len == 0) que.push(a);
            else{
                for(int i = 0; i < len; ++ i ){
                    string tmp = a+num_map[digits[index]-'0'][i];
                    que.push(tmp);
                }
            }
        }
        index++;
        
    }
    while(!que.empty()) {res.push_back(que.front());que.pop();}
    return res;
}