LeetCode/LetterCombinations.java at main · karmaSince94/LeetCode · GitHub

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package com.vinay.practice.lc;

import java.util.ArrayList;
import java.util.List;

public class LetterCombinations {


	private static String[] keypad = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

	public static void main(String[] args) {
		// TODO Auto-generated method stub

		System.out.println(letterCombinations("23"));

	}

	public static List<String> letterCombinations(String digits) {


		List<String> combination = new ArrayList<>();
		if(digits.isEmpty()) {
			return combination;
		}
		getCombinations(digits, 0, new StringBuilder(), combination);
		return combination;

    }

	// backtracking
	public static void getCombinations(String digits, int index, StringBuilder sb, List<String> combination){

		if(digits.length() == index) {

			combination.add(sb.toString());
			return;

		}

		int digit = digits.charAt(index) - '0'; // convert '2' to 2

		for(int i=0; i<keypad[digit].length(); i++) {

			sb.append(keypad[digit].charAt(i));
			getCombinations(digits, index+1, sb, combination);
			sb.deleteCharAt(sb.length()-1);


		}


	}

}