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17. Letter Combinations of a Phone Number

Medium

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example 1:

Input: digits = “23”

Output: [“ad”,”ae”,”af”,”bd”,”be”,”bf”,”cd”,”ce”,”cf”]

Example 2:

Input: digits = “”

Output: []

Example 3:

Input: digits = “2”

Output: [“a”,”b”,”c”]

Constraints:

• 0 <= digits.length <= 4
• digits[i] is a digit in the range ['2', '9'].

Solution

using System.Text;

public class Solution {
    public IList<string> LetterCombinations(string digits) {
        if (string.IsNullOrEmpty(digits)) {
            return new List<string>();
        }
        string[] letters = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        IList<string> ans = new List<string>();
        StringBuilder sb = new StringBuilder();
        FindCombinations(0, digits, letters, sb, ans);
        return ans;
    }

    private void FindCombinations(
            int start, string nums, string[] letters, StringBuilder curr, IList<string> ans) {
        if (curr.Length == nums.Length) {
            ans.Add(curr.ToString());
            return;
        }
        for (int i = start; i < nums.Length; i++) {
            int n = int.Parse(nums[i].ToString());
            for (int j = 0; j < letters[n].Length; j++) {
                char ch = letters[n][j];
                curr.Append(ch);
                FindCombinations(i + 1, nums, letters, curr, ans);
                curr.Length--; // Equivalent to deleting the last character in StringBuilder
            }
        }
    }
}

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