LeetCode Top 100 Liked Questions 17. Letter Combinations of a Phone Number (Java版； Medium)

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LeetCode Top 100 Liked Questions 17. Letter Combinations of a Phone Number (Java版; Medium)

题目描述

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 List<String> letterCombinations(String digits) {
        HashMap<Character, 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");

        List<String> list = new ArrayList<>();
        if(digits.length()==0){
            return list;
        }
        StringBuilder sb = new StringBuilder();
        core(map, list, sb, digits.toCharArray(), 0);
        return list;
    }

    private void core(HashMap<Character, String> map, List<String> list, StringBuilder sb, char[] chs, int i){
        //base case
        if(i==chs.length){
            list.add(sb.toString());
            return;
        }
        //
        char ch = chs[i];
        for(char ch2 : map.get(ch).toCharArray()){
            sb.append(ch2);
            core(map, list, sb, chs, i+1);
            sb.deleteCharAt(sb.length()-1);
        }
    }
}

第一次做,递归, 回溯法? 深度优先遍历? 基础不牢: 匿名内部类? String和char要严格区分, str.charAt()返回的是char, 不是String!!! 改变现场, 恢复现场

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

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<>();
        if(digits==null||digits.length()==0)
            return res;
        //匿名内部类
        HashMap<Character, String> hm = new HashMap<Character, 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");
            }
        };
        StringBuilder sb = new StringBuilder();
        Core(digits, 0, sb, res, hm);
        return res;
    }
    public void Core(String digits, int index, StringBuilder sb, List<String> res, HashMap<Character, String> hm){
        if(index==digits.length()){
            res.add(sb.toString());
            return;
        }            
        String curr = hm.get(digits.charAt(index));
        for(int i=0; i<curr.length(); i++){
            sb.append(curr.charAt(i));
            Core(digits, index+1, sb, res, hm);
            //restore
            sb.deleteCharAt(sb.length()-1);
        }
    }
}

匿名内部类

利用了队列的优秀答案

public List<String> letterCombinations(String digits) {
    LinkedList<String> ans = new LinkedList<String>();
    if(digits.isEmpty()) return ans;
    String[] mapping = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    ans.add("");
    for(int i =0; i<digits.length();i++){
      int x = Character.getNumericValue(digits.charAt(i));
      while(ans.peek().length()==i){
        String t = ans.remove();
        for(char s : mapping[x].toCharArray())
          ans.add(t+s);
      }
    }
    return ans;
  }