LetterCombinations.java

package recursion_and_dynamic_programming;

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

/**
 * @Author: Wenhang Chen
 * @Description:给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。
 * 给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
 * @Date: Created in 13:06 11/18/2019
 * @Modified by:
 */
public class LetterCombinations {
    // 灵活使用HashMap
    Map<String, String> phone = new HashMap<String, String>() {
        {
            put("2", "abc");
            put("3", "def");
            put("4", "ghi");
            put("5", "jkl");
            put("6", "mno");
            put("7", "pqrs");
            put("8", "tuv");
            put("9", "wxyz");
        }
    };
    List<String> strList = new ArrayList<>();

    public void recursion(String combination, String next_digits) {
        if (next_digits.length() == 0) {
            strList.add(combination);
        } else {
            String digit = next_digits.substring(0, 1);
            String letters = phone.get(digit);
            for (int i = 0; i < letters.length(); i++) {
                String letter = phone.get(digit).substring(i, i + 1);
                recursion(combination + letter, next_digits.substring(1));
            }
        }
    }

    public List<String> letterCombinations(String digits) {
        if (digits != null && digits.length() != 0)
            recursion("", digits);
        return strList;
    }
}