[Leetcode 47] 17 Letter Combinations of a Phone Number

Problem:

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"].

Analysis:

This is a simple backtracking problem. Backtrack method view the solution sapce as a tree. And it will go down from the root(initial state) to the leaf(some solution) to build a solution for the problem. A general structure of a backtracking program is as follows:

void backtrack(p1, ..., pn) {

　　if (current solution is a well formed solution) {

　　　　//do something

　　　　return;

　　}

　　for every child of current node

　　　　backtrack()

}

With this in mind, the problem is easy to solve.

Code:

class Solution {
public:
            vector<string> table;
            vector<string> res;

    vector<string> letterCombinations(string digits) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (digits.size() == 0) 
            return vector<string>(1,"");
        
        res.clear();
        table.resize(10);
        table[2] = "abc";
        table[3] = "def";
        table[4] = "ghi";
        table[5] = "jkl";
        table[6] = "mno";
        table[7] = "pqrs";
        table[8] = "tuv";
        table[9] = "wxyz";    
        
        backtrack(digits, 0, "");
        return res;
    }
    
    void backtrack(string &s, int pos, string ss) {
        if (pos == s.size()) {
            res.push_back(ss);
            return ;
        }
        
        for (int i=0; i<table[s[pos] - '0'].size(); i++) {
            backtrack(s, pos+1, ss+table[s[pos] - '0'][i]);
        }
        
        return ;
    }
};

Attention:

There are many C++ skills used in this code, learn it.