String——No.17 Letter Combinations of a Phone Number-优快云博客

本文介绍了一种算法，用于求解电话键盘上给定数字串的所有可能字母组合。通过递归方式实现，利用映射表将数字转换为对应的字母，进而生成所有可能的组合。

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Problem：

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.

Explanation：

找出拨号键盘上两个数字对应英文的所有两两组合。

My Thinking：

My Solution：

Optimum Thinking:

使用map将数字和字母对应关系存入，使用递归对数字对应字母进行组合。

Optimum Solution：

class Solution {
    private List<String> output=new ArrayList<String>();
    private Map<String,String> numletter=new HashMap<>();
    public List<String> letterCombinations(String digits) {
        
        numletter.put("2","abc");
        numletter.put("3","def");
        numletter.put("4","ghi");
        numletter.put("5","jkl");
        numletter.put("6","mno");
        numletter.put("7","pqrs");
        numletter.put("8","tuv");
        numletter.put("9","wxyz");
        if(digits.length()!=0)
            backtrack("",digits);
        return output;
    }
    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=numletter.get(digit);
            for (int i = 0; i < letters.length(); i++) {
                String letter = numletter.get(digit).substring(i, i + 1);
                backtrack(combination + letter, next_digits.substring(1));
            }
        }
    }
}