代码随想录算法训练营第二十五天|216. 组合总和 III、17. 电话号码的字母组合

 1 class Solution(object):
 2     def __init__(self):
 3         self.res = []
 4         self.sum_now = 0
 5         self.path = []
 6     def combinationSum3(self, k, n):
 7         """
 8         :type k: int
 9         :type n: int
10         :rtype: List[List[int]]
11         """
12         self.backtracking(k, n, 1)
13         return self.res
14 
15     def backtracking(self, k, n, start_num):
16         if self.sum_now > n:  # 剪枝
17             return
18         if len(self.path) == k:  # len(path)==k时不管sum是否等于n都会返回
19             if self.sum_now == n:
20                 self.res.append(self.path[:])
21             return
22         for i in range(start_num, 10 - (k - len(self.path)) + 1):
23             self.path.append(i)
24             self.sum_now += i
25             self.backtracking(k, n, i + 1)#递归
26             #回溯
27             self.path.pop()
28             self.sum_now -= i

 1 class Solution(object):
 2     def __init__(self):
 3         self.answers = []
 4         self.answer = ''
 5         self.letter_map = {
 6             '2': 'abc',
 7             '3': 'def',
 8             '4': 'ghi',
 9             '5': 'jkl',
10             '6': 'mno',
11             '7': 'pqrs',
12             '8': 'tuv',
13             '9': 'wxyz'
14         }
15     def letterCombinations(self, digits):
16         """
17         :type digits: str
18         :rtype: List[str]
19         """
20         if not digits:
21             return []
22         self.backtracking(digits, 0)
23         
24         return self.answers
25 
26     def backtracking(self, digits, index):
27         if index == len(digits):
28             self.answers.append(self.answer)
29             return
30 
31         letters = self.letter_map[digits[index]]
32         for letter in letters:
33             self.answer += letter
34             self.backtracking(digits, index+1)
35             self.answer = self.answer[:-1]  # 回溯
36             print(self.answer)