Leetcode回溯相关题目Python实现

1、46题，全排列 

https://leetcode-cn.com/problems/permutations/

class Solution(object):
    def permute(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        n = len(nums)
        results = []
        def backtrack(first = 0):
            if first == n:
                results.append(nums[:])
                return
            for i in range(first, n):
                nums[first], nums[i] = nums[i], nums[first]
                backtrack(first + 1)
                nums[first], nums[i] = nums[i], nums[first]
        backtrack()
        return results

2、47题，全排列二

https://leetcode-cn.com/problems/permutations-ii/

class Solution(object):
    def permuteUnique(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        n = len(nums)
        results = []
        def backtrack(first = 0):
            if first == n:
                results.append(nums[:])
                return
            s = set()
            for i in range(first, n):
                if nums[i] in s:
                    continue
                s.add(nums[i])
                nums[first], nums[i] = nums[i], nums[first]
                backtrack(first + 1)
                nums[first], nums[i] = nums[i], nums[first]
        backtrack()
        return results

3、51题，N皇后

https://leetcode-cn.com/problems/n-queens/

class Solution(object):
    def solveNQueens(self, n):
        """
        :type n: int
        :rtype: List[List[str]]
        """
        results = []
        result = ["." * n] * n
        l = set()
        c = set()
        x = set()
        y = set()

        def backtrack(first=0):
            if first == n:
                results.append(result[:])
                return
            for i in range(n):
                if i in l or first in c or i - first in x or i + first in y:
                    continue
                l.add(i), c.add(first), x.add(i - first), y.add(i + first)
                result[i] = first * "." + "Q" + (n - first - 1) * "."
                backtrack(first + 1)
                l.remove(i), c.remove(first), x.remove(i - first), y.remove(i + first)
                result[i] = n * "."
        backtrack()
        return results

4、52题，N皇后二

https://leetcode-cn.com/problems/n-queens-ii/

class Solution(object):
    def totalNQueens(self, n):
        """
        :type n: int
        :rtype: int
        """
        self.results = 0
        l = set()
        c = set()
        x = set()
        y = set()

        def backtrack(first=0):
            if first == n:
                self.results += 1
                return
            for i in range(n):
                if i in l or first in c or i - first in x or i + first in y:
                    continue
                l.add(i), c.add(first), x.add(i - first), y.add(i + first)
                backtrack(first + 1)
                l.remove(i), c.remove(first), x.remove(i - first), y.remove(i + first)
        backtrack()
        return self.results