Leetcode:17. Letter Combinations of a Phone Number

Description

解题思路

1.DFS

class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> result;        string save;        if (digits.size() == 0) return vector<string>();        int convert[10] = {0, 3, 6, 9, 12, 15, 19, 22, 26};        dfs(result, save, digits, 0, digits.size(), convert);        return result;    }    void dfs(vector<string> &str, string save, string dig, int now, int size, int *convert){        if(now == size){            str.push_back(save);            return;        }        int tmp = dig[now] - '0' - 2;//change '2' to 0, '3' to 1...        for(int i = convert[tmp]; i < convert[tmp+1]; i++){            save.push_back('a' + i);//ex: a,b,c | d,e,f            dfs(str, save, dig, now + 1, size, convert);            save.pop_back();        }    }};

2.采用迭代方法求解，以123为例

class Solution {public:    vector<string> letterCombinations(string digits) {        if (digits.size() == 0) return vector<string>();        static const vector<string> v = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};        vector<string> result;        result.push_back("");        for (int i = 0; i < digits.size(); i++) {            int num = digits[i]-'0';            if (num > 9 || num < 0) break;            string s = v[num];            vector<string> t;            for (int j = 0; j < s.size(); j++) {                for (int k = 0; k < result.size(); k++) {                    t.push_back(result[k]+s[j]);                }            }            result.swap(t);        }        return result;    }};