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.
AC代码：
class Solution(object): def letterCombinations(self, digits): if not digits: return [] ret_list = [] letters_list = (‘0‘, ‘1‘, ‘abc‘, ‘def‘, ‘ghi‘, ‘jkl‘, ‘mno‘, ‘pqrs‘, ‘tuv‘, ‘wxyz‘) def backtracing(letters_list, digits, ret_list, index, current): # boundary condition, recurse to the deepest if index == len(digits): ret_list.append(current) return for v in letters_list[int(digits[index])]: backtracing(letters_list, digits, ret_list, index + 1, current + v) backtracing(letters_list, digits, ret_list, 0, ‘‘) return ret_list
此方法用了经典的回溯法进行递归查找。
还可以用迭代的方式进行查找：
class Solution(object): # version iteration, use reduce def letterCombinations(self, digits): if not digits: return [] letters_list = (‘0‘, ‘1‘, ‘abc‘, ‘def‘, ‘ghi‘, ‘jkl‘, ‘mno‘, ‘pqrs‘, ‘tuv‘, ‘wxyz‘) return reduce(lambda ret_list, index: [e + v for e in ret_list for v in letters_list[int(index)]], digits, [‘‘])
利用reduce函数，其核心代码只有一行，注意reduce第三个参数一定要给初始值 [‘‘]。
随着第一种方法递归深度的增加，效率会变得越来越差，所以推荐第二种方法，既优雅效率也高。
17. Letter Combinations of a Phone Number
原文：http://www.cnblogs.com/zhuifengjingling/p/5215564.html