17. Letter Combinations of a Phone Number -Medium

Question

Given a digit string, 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.

给出一个包含数字的字符串，请你根据每个数字对应于手机按钮的字母返回所有的组合

Example

Input:Digit string “23”
Output: [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].

Solution

• 回溯解。首先我们先对应每个数字对应的字母String[] S = {“”, “”, “abc”, “def”, “ghi”, “jkl”, “mno”, “pqrs”, “tuv”, “wxyz”};，这样对于输入的任意1-9的数字i，S[i]就是i所对应的字符。然后我们对每个字母进行遍历，将其添加到组合中，并更新下一个输入的数字进行回溯即可

  public class Solution {
      private String[] S = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
      public List letterCombinations(String digits) {
          List res = new ArrayList<>();
          if(digits.equals("")) return res;
          backtracking(digits, 0, res, "");
          return res;
      }
  
      public void backtracking(String digits, int start, List res, String temp){
          if(temp.length() > digits.length()) return;
          if(temp.length() == digits.length()){
              res.add(temp);
              return;
          }
  
          // 获得当前数字对应的字符
          String letters = S[digits.charAt(start) - '0'];
          // 分别添加每个字母
          for(int i = 0; i < letters.length(); i++){
              // 对每个字母，接下来更新下一个数字的索引，进行回溯
              backtracking(digits, start + 1, res, temp + letters.charAt(i));
          }
      }
  }