[Leetcode] 17. Letter Combinations of a Phone Number

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本文介绍了一种算法，用于根据电话按键上的数字到字母映射，生成所有可能的字母组合。通过迭代方法，针对输入的数字字符串，返回其对应的所有可能的字母组合。

Problem:

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.

思路：
本题其实就是列举若干个集合间组合的所有情况。可以用迭代的方法完成。迭代的方法如下：令当前数字与之前的所有情况result list进行组合，然后再把组合得到的结果赋值给result list，实现迭代。

Solution:

class Solution(object):
    digitlist = []
    count = 97
    i = 0
    for i in xrange(10):
        tmpl = []
        if i == 0 or i == 1:
            digitlist.append(tmpl)
            continue
        elif i ==7 or i == 9:
            num = 4
        else:
            num = 3
        j = 0
        for j in xrange(num):
            tmpl.append(chr(count))
            count += 1
        digitlist.append(tmpl)

    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if len(digits) == 0 :
            return []
        result = [""]
        for s in digits:
            num = int(s)
            if num >=2 and num <= 9:
                tmplist = []
            else:
                continue
            for d in self.digitlist[num]:
                for item in result:
                    tmplist.append(item+d)
            result = tmplist
        return result