import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

class Solution {
    public List<String> letterCombinations(String digits) {

    	if (digits == null || digits.length() == 0) {
    		return new ArrayList<String>();
    	}

        String[] MAPPING = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

        List<List<String>> levels = new ArrayList<>();
        levels.add(new ArrayList<String>());
        levels.add(new ArrayList<String>());
        int first = 0;
        int second = 1;
        levels.get(first).add("");

        for (int i = 0; i < digits.length(); i++) {
        	int key = Character.getNumericValue(digits.charAt(i));
        	char[] charArr = MAPPING[key].toCharArray();
        	List<String> curr = levels.get(first);
        	List<String> next = levels.get(second);

        	for (char ch : charArr) {
        		for (String item : curr) {
        			next.add(item + ch);
        		}
        	}

        	curr.clear();

        	first = first ^ 1;
        	second = second ^ 1;
        }

        return levels.get(first);
    }
}