【17】17. Letter Combinations of a Phone Number

17. Letter Combinations of a Phone Number

• Total Accepted: 125773
• Total Submissions: 384342
• Difficulty: Medium
• Contributors: Admin

 

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.

Solution1: (dfs) Recursive

class Solution {
public:
    vector<string> n2l = {" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if(digits.size() == 0){
            return res;
        }
        string level;
        dfs(digits, res, level, 0);
        return res;
    }
    
    void dfs(string digits, vector<string>& res, string& level, int start){
        //if(start == digits.size()){//base case 不对
        if(level.size() == digits.size()){
            res.push_back(level);
            return;
        }
        for(int i = start; i < digits.size(); i++){
            for(int j = 0; j < n2l[digits[i] - '0'].size(); j++){
                level += n2l[digits[i] - '0'][j];
                dfs(digits, res, level, i + 1);
                //level -= n2l[digits[i] - '0'][j];
                level.resize(level.size() - 1);
            }
        }
        //return res;
    }
};

Solution 2: Iterative