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Update 0017.电话号码的字母组合.md

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jianghongcheng
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problems/0017.电话号码的字母组合.md
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@ -285,98 +285,153 @@ class Solution {
```
## Python
**回溯**
回溯
```python
class Solution:
def __init__(self):
self.answers: List[str] = []
self.answer: str = ''
self.letter_map = {
'2': 'abc',
'3': 'def',
'4': 'ghi',
'5': 'jkl',
'6': 'mno',
'7': 'pqrs',
'8': 'tuv',
'9': 'wxyz'
}
def letterCombinations(self, digits: str) -> List[str]:
self.answers.clear()
if not digits: return []
self.letterMap = [
"", # 0
"", # 1
"abc", # 2
"def", # 3
"ghi", # 4
"jkl", # 5
"mno", # 6
"pqrs", # 7
"tuv", # 8
"wxyz" # 9
]
self.result = []
self.s = ""
def backtracking(self, digits, index):
if index == len(digits):
self.result.append(self.s)
return
digit = int(digits[index]) # 将索引处的数字转换为整数
letters = self.letterMap[digit] # 获取对应的字符集
for i in range(len(letters)):
self.s += letters[i] # 处理字符
self.backtracking(digits, index + 1) # 递归调用，注意索引加1，处理下一个数字
self.s = self.s[:-1] # 回溯，删除最后添加的字符
def letterCombinations(self, digits):
if len(digits) == 0:
return self.result
self.backtracking(digits, 0)
return self.answers
def backtracking(self, digits: str, index: int) -> None:
# 回溯函数没有返回值
# Base Case
if index == len(digits): # 当遍历穷尽后的下一层时
self.answers.append(self.answer)
return
# 单层递归逻辑
letters: str = self.letter_map[digits[index]]
for letter in letters:
self.answer += letter # 处理
self.backtracking(digits, index + 1) # 递归至下一层
self.answer = self.answer[:-1] # 回溯
return self.result
```
**回溯简化**
回溯精简（版本一）
```python
class Solution:
def __init__(self):
self.answers: List[str] = []
self.letter_map = {
'2': 'abc',
'3': 'def',
'4': 'ghi',
'5': 'jkl',
'6': 'mno',
'7': 'pqrs',
'8': 'tuv',
'9': 'wxyz'
}
def letterCombinations(self, digits: str) -> List[str]:
self.answers.clear()
if not digits: return []
self.backtracking(digits, 0, '')
return self.answers
self.letterMap = [
"", # 0
"", # 1
"abc", # 2
"def", # 3
"ghi", # 4
"jkl", # 5
"mno", # 6
"pqrs", # 7
"tuv", # 8
"wxyz" # 9
]
self.result = []
def backtracking(self, digits: str, index: int, answer: str) -> None:
# 回溯函数没有返回值
# Base Case
if index == len(digits): # 当遍历穷尽后的下一层时
self.answers.append(answer)
return
# 单层递归逻辑
letters: str = self.letter_map[digits[index]]
def getCombinations(self, digits, index, s):
if index == len(digits):
self.result.append(s)
return
digit = int(digits[index])
letters = self.letterMap[digit]
for letter in letters:
self.backtracking(digits, index + 1, answer + letter) # 递归至下一层 + 回溯
self.getCombinations(digits, index + 1, s + letter)
def letterCombinations(self, digits):
if len(digits) == 0:
return self.result
self.getCombinations(digits, 0, "")
return self.result
```
**使用itertools**
回溯精简（版本二）
```python
class Solution:
def letterCombinations(self, digits: str) -> List[str]:
import itertools
if not digits:
return list()
phoneMap = {
"2": "abc",
"3": "def",
"4": "ghi",
"5": "jkl",
"6": "mno",
"7": "pqrs",
"8": "tuv",
"9": "wxyz",
}
def __init__(self):
self.letterMap = [
"", # 0
"", # 1
"abc", # 2
"def", # 3
"ghi", # 4
"jkl", # 5
"mno", # 6
"pqrs", # 7
"tuv", # 8
"wxyz" # 9
]
def getCombinations(self, digits, index, s, result):
if index == len(digits):
result.append(s)
return
digit = int(digits[index])
letters = self.letterMap[digit]
for letter in letters:
self.getCombinations(digits, index + 1, s + letter, result)
def letterCombinations(self, digits):
result = []
if len(digits) == 0:
return result
self.getCombinations(digits, 0, "", result)
return result
groups = (phoneMap[digit] for digit in digits)
return ["".join(combination) for combination in itertools.product(*groups)]
```
回溯优化使用列表
```python
class Solution:
def __init__(self):
self.letterMap = [
"", # 0
"", # 1
"abc", # 2
"def", # 3
"ghi", # 4
"jkl", # 5
"mno", # 6
"pqrs", # 7
"tuv", # 8
"wxyz" # 9
]
def getCombinations(self, digits, index, path, result):
if index == len(digits):
result.append(''.join(path))
return
digit = int(digits[index])
letters = self.letterMap[digit]
for letter in letters:
path.append(letter)
self.getCombinations(digits, index + 1, path, result)
path.pop()
def letterCombinations(self, digits):
result = []
if len(digits) == 0:
return result
self.getCombinations(digits, 0, [], result)
return result
```
## Go