Letter Combinations of a Phone Number

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本文介绍了一种使用深度优先搜索(DFS)算法来解决电话号码字符串转换为所有可能的字母组合的问题。通过构建映射表将数字与对应的字母关联起来，并利用递归方式生成所有可能的组合。

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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.

Input:Digit string "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.

Analysis: DFS.

Note: The toString() method of StringBuilder will return a newly allocated String object. In other word, it is a "deep copy" of the content in the StringBuilder instance.

toString
```
public String toString()
```
Returns a string representing the data in this sequence. A new String object is allocated and initialized to contain the character sequence currently represented by this object. This String is then returned. Subsequent changes to this sequence do not affect the contents of the String.

Specified by:
toString in interface CharSequence
Returns:
a string representation of this sequence of characters.

public class Solution {
    public HashMap<Character, char[]> map = new HashMap<Character, char[]>();
    
    public void constructMap() {
        this.map.put('2', "abc".toCharArray());
        this.map.put('3', "def".toCharArray());
        this.map.put('4', "ghi".toCharArray());
        this.map.put('5', "jkl".toCharArray());
        this.map.put('6', "mno".toCharArray());
        this.map.put('7', "pqrs".toCharArray());
        this.map.put('8', "tuv".toCharArray());
        this.map.put('9', "wxyz".toCharArray());
    }
    
    public void letterCombinations(String digits, int index, ArrayList<String> res, StringBuilder tem) {
        if(index == digits.length()) {
            res.add(tem.toString());
            return;
        }
        char[] current = map.get(digits.charAt(index));
        for(int i=0; i<current.length; i++) {
            tem.append(current[i]);
            letterCombinations(digits, index+1, res, tem);
            tem.deleteCharAt(tem.length()-1);
        }
        return;
    }
    
    public ArrayList<String> letterCombinations(String digits) {
        ArrayList<String> res = new ArrayList<String>();
        StringBuilder tem = new StringBuilder();
        constructMap();
        letterCombinations(digits, 0, res, tem);
        return res;
    }
}