[英]Python permutations with condition (backtracking)
I want to solve a problem using backtracking. 我想使用回溯解决问题。 As in... I'm given a list of numbers and I want to find all the possible permutations that respect a given condition, using backtracking. 就像在...中,我得到了一个数字列表,我想使用回溯找到尊重给定条件的所有可能排列。
I have the code for generating a list of permutations but it's not helping cause I can't check each permutation individually before adding it to the list so it's not backtracking, it's just recursive. 我有用于生成排列列表的代码,但它无济于事,因为我无法在将每个排列添加到列表之前单独检查每个排列,因此它不会回溯,只是递归的。 I also understand the way backtracking works for: permutations from 0 to x but not for a list... 我也了解回溯的工作方式:从0到x的排列,但不是列表的排列...
This is my permutation list generator 这是我的排列列表生成器
def permutare(self, lista):
if len(lista) == 1:
return [lista]
res = []
for permutation in self.permutare(lista[1:]):
for i in range(len(lista)):
res.append(permutation[:i] + lista[0:1] + permutation[i:])
return res
Works but not helping me. 有效,但无济于事。 I tried inserting the validation somewhere in there but nowhere works.. I tried all the permutation algorithms I could find. 我尝试将验证插入到某处,但无处工作。.我尝试了所有可以找到的置换算法。 I need one with backtracking 我需要一个回溯
Any idea/algorithm/pseudocode for backtracking permutations with conditions? 对条件回溯排列有任何想法/算法/伪代码吗?
Here's a solution that uses backtracking by swapping elements in the list. 这是一个通过交换列表中的元素来使用回溯的解决方案。
The basic idea is: 基本思想是:
Code: 码:
def swap(lista, idx1, idx2):
temp = lista[idx1]
lista[idx1] = lista[idx2]
lista[idx2] = temp
def valid():
return True
def permutare(lista, start):
if start >= len(lista):
if valid():
return [list(lista)]
output = []
for idx in xrange(start, len(lista)):
swap(lista, start, idx)
output.extend(permutare(lista, start + 1))
swap(lista, start, idx) # backtrack
return output
print len(permutare(['a','b','c'], 0))
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