Letter Combinations of a Phone Number

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.

深度优先搜索

复杂度

时间 O(N) 空间 O(N) 递归栈空间

思路

首先建一个表，来映射号码和字母的关系。然后对号码进行深度优先搜索，对于每一位，从表中找出数字对应的字母，这些字母就是本轮搜索的几种可能。

注意

• 用StringBuilder构建临时字符串

• 当临时字符串为空时，不用将其加入结果列表中

代码

public class Solution {
    
    List<String> res;
    
    public List<String> letterCombinations(String digits) {
        // 建立映射表
        String[] table = {" ", " ", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        StringBuilder tmp = new StringBuilder();
        res = new LinkedList<String>();
        helper(table, 0, tmp, digits);
        return res;
    }
    
    private void helper(String[] table, int idx, StringBuilder tmp, String digits){
        if(idx == digits.length()){
            // 找到一种结果，加入列表中
            if(tmp.length()!=0) res.add(tmp.toString());
        } else {
            // 找出当前位数字对应可能的字母
            String candidates = table[digits.charAt(idx) - '0'];
            // 对每个可能字母进行搜索
            for(int i = 0; i < candidates.length(); i++){
                tmp.append(candidates.charAt(i));
                helper(table, idx+1, tmp, digits);
                tmp.deleteCharAt(tmp.length()-1);
            }
        }
    }
}