LeetCode 17 电话号码的字母组合（难度：Medium）

落日映苍穹つ 2022-01-31 01:19 184阅读 0赞

**题目大意：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。**

![200px-Telephone-keypad2.svg.png][]

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"].

解决方案：回溯法

package pers.leetcode;
    
    import java.util.ArrayList;
    import java.util.HashMap;
    import java.util.List;
    import java.util.Map;
    
    /**
     * Letter Combinations of a Phone Number（电话号码的字母组合）
     * 难易程度：Medium
     *
     * @author jinghui.zhu
     * @date 2019/5/15 9:54
     */
    public class LeetCode17 {
    
        public static void main(String[] args) {
            LeetCode17 leetCode17 = new LeetCode17();
            String phone = "23";
            System.out.println(leetCode17.letterCombinations(phone));
    
        }
    
        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> output = new ArrayList<>();
    
        public void backtrack(String combination, String next_digits){
            if (next_digits.length() == 0){
                output.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);
                    backtrack(combination + letter, next_digits.substring(1));
                }
            }
        }
    
        public List<String> letterCombinations(String digits){
            if (digits.length() != 0){
                backtrack("", digits);
            }
            return output;
        }
    }

[200px-Telephone-keypad2.svg.png]: /images/20220131/f494b547d77b4bc5880bff9fb0038095.png