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"].
Solution:
Use a queue is the key.
class Solution {
public:
vector<string> letterCombinations(string digits) {
vector<string> result;
if (digits.length() == 0) {
return result;
}
queue<string> combinations;
combinations.push("");
const vector<string> digitToLetters = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
for (int i = 0; i < digits.size(); i++) {
// count is number of strings from upper level with same length
int count = combinations.size();
for (int j = 0; j < count; j++) {
const auto& front = combinations.front();
const auto& letters = digitToLetters[digits[i] - '0'];
for (int k = 0; k < letters.length(); k++) {
cout << front + letters[k] << endl;
if (i == digits.size() - 1) { // output as final string when reached last digit
result.push_back(front + letters[k]);
} else {
combinations.push(front + letters[k]);
}
}
combinations.pop();
}
}
return result;
}
};