Java recursive backtracking simple

  • 0

    As simple as recursive backtracking gets. Recursively add characters to a cumulative string, backtracking when the string has been filled out to the maximum number of digits.

    class Solution {
        private ArrayList<String> master = new ArrayList<String>();
        public List<String> letterCombinations(String digits) {
            if (digits.equals("")) {
                return master;
            HashMap<String, String> map = new HashMap<>();
            map.put("2", "abc");
            map.put("3", "def");
            map.put("4", "ghi");
            map.put("5", "jkl");
            map.put("6", "mno");
            map.put("7", "pqrs");
            map.put("8", "tuv");
            map.put("9", "wxyz");
            combine("", digits, map);
            return master;
        private void combine(String current, String remaining, HashMap<String, String> map) {
            if ("".equals(remaining)) {
            String letters = map.get(remaining.substring(0,1));
            for (int i = 0; i < letters.length(); i++) {
                current += letters.substring(i, i + 1);
                combine(current, remaining.substring(1), map);
                current = current.substring(0, current.length() - 1);

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