(算法分析Week10)Letter Combinations of a Phone Number[Medium]

17. Letter Combinations of a Phone Number[Medium]

题目来源

Description

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.

Solution

一个不停迭代的过程，粗暴一点就用嵌套循环，可以过。
用一个字符串数组存储结果，每次取接下来的那个数字，将其对应的字符，利用循环，分别接在原来的字符串数组后面即可。
数字对应字符串（键盘数字英文对应）也用一个字符串数组存储。
细心一点处理边界情况和空串。

Complexity analysis

O(nm)
n = digits.size
m = result.size

Code

class Solution {
public:
    vector<string> letterCombinations(string digits) {

        vector<string> result;
        if (digits == "") //如果强行先push_back空串，当输入也为空时，则错误，所以要特殊处理
            return result;
        result.push_back(""); //一开始没有注意这里，导致最内层循环无法进入
        string tel[10] = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        for (int i = 0; i < digits.size(); i++) {
            string str = tel[digits[i] - '0'];
            vector<string> temp;
            for (int j = 0; j < str.size(); j++) {
                for (int k = 0; k < result.size(); k++) {
                    temp.push_back(result[k] + str[j]);
                }
            }
            result = temp;
        }
        return result;
    }

};

Result