leetcode-17. Letter Combinations of a Phone Number

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本文介绍了一种使用深度优先搜索(DFS)递归算法解决LeetCode上的经典问题——电话号码的字母组合的方法。通过给定的数字字符串，返回所有可能的字母组合。详细解释了如何迭代地生成每个可能的组合，并提供了完整的C++代码实现。

leetcode-17. 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"].

按照电话数字把对应的字母写出来
用迭代的方法，

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if(digits.empty())
            return res;
        string dict[] = {"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        letterCombinationsDFS(digits, dict, 0, "", res);
        return res;
    }

    void letterCombinationsDFS(string digits, string dict[], int level, string out, vector<string> &res){
        if(level == digits.size())
            res.push_back(out);
        else{
            string str= dict[digits[level] - '2'];     //注意是'2',不是2
            for(int i = 0; i < str.size(); ++i){
                out.push_back(str[i]);
                letterCombinationsDFS(digits, dict, level + 1, out, res);
                out.pop_back();                     //为什么要pop_back？
            }
        }

    }

};