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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

解题思路：

方法一：暴力穷举，只需要依次遍历每个数字对应的可能字符，复杂度爆表，具体实现不考虑，只是一种效率低的思路

方法二：正确的解法应该使用回溯，可以把题目想象成一个N行的数组，N表示digits的位数，每一行根据数字对应的字符，可能有0种、3种、4种选择。有点像N皇后问题，也像树的深度优先搜索，因此使用回溯。具体解释见代码。

class Solution {
public:
    vector letterCombinations(string digits) {
        string str[] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        vector result;
        string temp;
        if (digits.size() == 0) return result;
        Combinations(digits, 0, str, temp, result);
        return result;
    }
    void Combinations(string digits, int index, string str[], string temp, vector &result)
    {     //index用来指示遍历到数字的末尾
        if (index == digits.size())
        {
            result.push_back(temp);
            return;
        }
        char s = digits[index]; //digits中某个数字
        for (int i = 0; i < (str[s-'0']).size(); ++i) //这个数字有这么多种可能的选择
        {
            temp += str[s-'0'][i];
            Combinations(digits, index+1, str, temp, result);
            temp.pop_back(); //C++11 string支持pop_back()，回溯的时候要把上次加进去的删掉
        }
    }
};