17. Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

class Solution {
    public char [][] dist;
    public int []num;
    public void dfs(char[] now, int pos, List<String> ans, String digits) {
        if (pos == digits.length()) {
            char[] temp = new char[pos];
            for (int i = 0; i < pos; ++i)
                temp[i] = now[i];
            ans.add(String.valueOf(temp));
            return;
        }
        char c = digits.charAt(pos);
        int id = c - '2';
        for (int i = 0; i < num[id]; ++i) {
            now[pos] = dist[id][i];
            dfs(now, pos + 1, ans, digits);
        }
        
        
    }
    
    public void init() {
        int []temp = new int[]{3, 3, 3, 3, 3, 4, 3, 4};
        dist = new char[8][5];
        int sum = 0;
        for (int i = 0; i < 8; ++i) {
            sum += i== 0 ? 0 : temp[i - 1];
            for (int j = 0; j < temp[i]; ++j) {
                dist[i][j] = (char)(j  + 97 + sum);
            }
        }
        num = temp;
    }
    public List<String> letterCombinations(String digits) {
        List<String> ans = new ArrayList<>();
        if (digits.length() == 0) return ans;
        int n = digits.length();
        char []now = new char[n + 1];
        init();
       
        
        dfs(now, 0, ans, digits);
        return ans;
    }
}