Letter Combinations of a Phone Number Solutions in C++

// Source : https://oj.leetcode.com/problems/letter-combinations-of-a-phone-number/
// Author : Hao Chen
// Date   : 2014-07-17



#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;

vector<string> letterCombinations(string digits) {
    char  phone[10][4]={ {' ',  '\0', '\0', '\0' }, //0
                         {'\0', '\0', '\0', '\0' }, //1
                         {'a',  'b',  'c',  '\0' }, //2
                         {'d',  'e',  'f',  '\0' }, //3
                         {'g',  'h',  'i',  '\0' }, //4
                         {'j',  'k',  'l',  '\0' }, //5
                         {'m',  'n',  'o',  '\0' }, //6
                         {'p',  'q',  'r',  's'  }, //7
                         {'t',  'u',  'v',  '\0' }, //8
                         {'w',  'x',  'y',  'z'  }  //9
                       };

    vector<string> result;
    if (digits.size()<=0){
        result.push_back("");
        return result;
    }
    for( int i=0; i<digits.size(); i++ ) {
        if (!isdigit(digits[i])) {
            vector<string> r;
            return r;
        } 
        int d = digits[i] - '0';
        if (result.size()<=0){
            for( int j=0; j<4 && phone[d][j]!='\0'; j++ ){
                string s;
                s += phone[d][j];
                result.push_back(s);
            }
            continue;
        }
        vector<string> r;
        for (int j=0; j<result.size(); j++){
            for( int k=0; k<4 && phone[d][k]!='\0'; k++ ){
                string s = result[j] + phone[d][k];
                //sort(s.begin(), s.end());
                r.push_back(s); 
            }
        }
        result = r;
    }
    //sort(result.begin(), result.end());

    return result; 
}

void printVector(vector<string>& ss){
    cout << "{ ";
    for(int i=0; i<ss.size(); i++){
        if (i>0) cout << ", "; 
        cout << ss[i];
    }
    cout << " }" << endl;
}

int main(int argc, char**argv)
{
    string s="23";
    if (argc>1){
        s=argv[1];
    }
    vector<string> ss = letterCombinations(s);
    printVector(ss);
    return 0;
}

//
//  Solution.h
//  Test
//
//  Created by pezy on 12/27/14.
//  Copyright (c) 2014 pezy. All rights reserved.
//

#ifndef Test_Solution_h
#define Test_Solution_h

#include <vector>
#include <string>
#include <array>

using std::vector; using std::string; using std::array;

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> ret{""};
        array<string, 10> map{"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        for (auto digit : digits) {
            vector<string> tmp;
            for (auto c : map[digit - '0'])
                for (const auto &pre : ret)
                    tmp.push_back(pre + c);
            ret = tmp;
        }
        return ret;
    }
};

#endif

/// Source : https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/
/// Author : liuyubobobo
/// Time   : 2017-11-18

#include <iostream>
#include <vector>
#include <string>
#include <cassert>

using namespace std;

/// Backtracking
/// Time Complexity: O(2^len(s))
/// Space Complexity: O(len(s))
class Solution {

private:
    const string letterMap[10] = {
            " ",    //0
            "",     //1
            "abc",  //2
            "def",  //3
            "ghi",  //4
            "jkl",  //5
            "mno",  //6
            "pqrs", //7
            "tuv",  //8
            "wxyz"  //9
    };

    vector<string> res;

    void findCombination(const string &digits, int index, const string &s){

        if(index == digits.size()){
            res.push_back(s);
            return;
        }

        char c = digits[index];
        assert(c >= '0' && c <= '9' && c != '1');
        string letters = letterMap[c - '0'];
        for(int i = 0 ; i < letters.size() ; i ++)
            findCombination(digits, index+1, s + letters[i]);

        return;
    }

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

        res.clear();
        if(digits == "")
            return res;

        findCombination(digits, 0, "");

        return res;
    }
};


void printVec(const vector<string>& vec){
    for(string s: vec)
        cout << s << endl;
}

int main() {

    printVec(Solution().letterCombinations("234"));

    return 0;
}