【leetcode】17. Letter Combinations of a Phone Number

题目描述：

Given a digit string, return all possible letter combinations that the number could represent.

解题分析：

回溯法的典型应用，用一个数据结构表示出按键与其表示字母的对应关系，直接用回溯法做即可。

具体代码：

public class Solution {
    private static List<String> results=new ArrayList<String>();
    private static String[] array= {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    public static List<String> letterCombinations(String digits) {
            //String[] array= {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
            results=new ArrayList<String>();
            if(digits==null||digits.length()==0)
                return results;
            StringBuilder sb = new StringBuilder();
            
            fun(digits,sb,0);
            return results;
    }

    private static void fun(String digits, StringBuilder sb,
            int i) {
        if(i==digits.length()-1){
            char ch = digits.charAt(i);
            int index = ch-'0';
            for(int j=0;j<array[index].length();j++){
                sb.append(array[index].charAt(j));
                results.add(sb.toString());
                sb.deleteCharAt(sb.length()-1);
            }
            return;
        }
        char ch = digits.charAt(i);
        int index = ch-'0';
        for(int j=0;j<array[index].length();j++){
            sb.append(array[index].charAt(j));
            fun(digits, sb, i+1);
            sb.deleteCharAt(sb.length()-1);
        }
    }
}