17. Letter Combinations of a Phone Number

 

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "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.

Accepted

467,028

Submissions

1,073,741

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> ret;
        if(digits==""){
        	return ret;
        }
        ret.push_back("");
        vector<string> dict(10);
        dict[2]="abc";
        dict[3]="def";
        dict[4]="ghi";
        dict[5]="jkl";
        dict[6]="mno";
        dict[7]="pqrs";
        dict[8]="tuv";
        dict[9]="wxyz";
        for(int i=0;i<digits.size();i++){//输入的长度作为第一层循环
        	int size=ret.size();
        	for(int j=0;j<size;j++){//中介的长度cur作为第二层循环
        		string cur=ret[0];
        		ret.erase(ret.begin());
        		for(int k=0;k<dict[digits[i]-'0'].size();k++){//第三层循环字符运算
        			ret.push_back(cur+dict[digits[i]-'0'][k]);
        		}
        	}
        }
        return ret;
    }
};