Leetcode 17. Letter Combinations of a Phone Number

题目：

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.

Example:

Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

 

题意：

如图手机按键上代表了几个字母，给定一串整数，请列出它们对应的字母所能组成的所有字母串。

 

解题：

我们可以根据整数串，建立一棵树，这棵树我们可以用DFS或者BFS来找到所有解

DFS的办法：

 

def letterCombinations(self, digits):
    if not digits:
        return []
    dic = {"2":"abc", "3":"def", "4":"ghi", "5":"jkl", "6":"mno", "7":"pqrs", "8":"tuv", "9":"wxyz"}
    res = []
    self.dfs(digits, dic, 0, "", res)
    return res
    
def dfs(self, digits, dic, index, path, res):
    if len(path) == len(digits):
        res.append(path)
        return 
    for i in xrange(index, len(digits)):
        for j in dic[digits[i]]:
            self.dfs(digits, dic, i+1, path+j, res)

BFS：

 

class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> list = new LinkedList<>();
        if(digits == null || digits.length() == 0) return list;
        list.add("");
        char[][] map = {{},{},{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};
        for(int i = 0;i< digits.length();i++){
            List<String> nextList = new LinkedList<>();
            int num = digits.charAt(i) - '0';
            for(String s: list){
                for(int k = 0;k< map[num].length;k++){
                    nextList.add(s+map[num][k]);
                }
            }
            list = nextList;
        }
        return list;
        
        
    }
}