题解 | #电话号码的字母组合#

import java.util.*;


public class Solution {

    private ArrayList<String> res = new ArrayList<>();
    private Map<Character, String> numMap = new HashMap<>();

    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param num string字符串
     * @return string字符串ArrayList
     */
    public ArrayList<String> phoneNumber (String num) {
        // write code here
        if (num == null || num.length() < 1) {
            return res;
        }
        numMap.put('2', "abc");
        numMap.put('3', "def");
        numMap.put('4', "ghi");
        numMap.put('5', "jkl");
        numMap.put('6', "mno");
        numMap.put('7', "pqrs");
        numMap.put('8', "tuv");
        numMap.put('9', "wxyz");
        backtrack(num, 0, new ArrayList<>());
        return res;
    }

    private void backtrack(String num, int start, List<Character> list) {
        if (start == num.length() && list.size() == num.length()) {
            res.add(list2Str(list));
            return;
        }
        String alpha = numMap.get(num.charAt(start));
        for (int i = 0; i < alpha.length(); i++) {
            list.add(alpha.charAt(i));
            backtrack(num, start + 1, list);
            list.remove(list.size() - 1);
        }
    }

    private String list2Str(List<Character> list) {
        StringBuilder sb = new StringBuilder();
        for (char ch : list) {
            sb.append(ch);
        }
        return sb.toString();
    }
}