Letter Combinations of a Phone Number

1、一个简单的动态规划；

lettercombination（string digits）
{
    help(digits,i,len);
}
void help(string digits,int i,int len)
{
    if(i==len)
    {
        str[len]='\0';
        string temp=str;
        res.push_back(temp);
        return;
    }

    int j=digits[i]-'0';
    for(int j=0;j<num[i].size();j++)
    {
        str[i]=num[i][j];
        help(digits,i+1,len);
    }
}

题目：
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.

思路：简单的动态规划思想
代码：

class Solution {
public:
   vector<string> res;
    string num[10];
    char   str[1000];
    vector<string> letterCombinations(string digits) {
        int len=digits.length();
        res.clear();
        num[2]="abc";
        num[3]="def";
        num[4]="ghi";
        num[5]="jkl";
        num[6]="mno";
        num[7]="pqrs";
        num[8]="tuv";
        num[9]="wxyz";
        help(digits,0,len);
        return res;
    }
    void help(string &digits,int i,int len)
    {


        if(i==len)
        {
            str[len]='\0';
            string temp=str;
            res.push_back(temp);
            return;
        }

        int n=digits[i]-'0';
        for(int j=0;j<num[i].size();j++)
        {
            str[i]=num[n][j];
            help(digits,i+1,len);
        }
    }
};