leetcode|活字印刷

{AAB}

class Solution:
    def __init__(self):
        self.path = ''
        self.paths = []

    def numTilePossibilities(self, nums):
        '''
        因为本题排列是有序的，这意味着同一层的元素可以重复使用，但同一树枝上不能重复使用(usage_list)
        所以处理排列问题每层都需要从头搜索，故不再使用start_index
        '''
        usage_list = [False] * len(nums)
        self.backtracking(nums, usage_list)
        return len(self.paths)

    def backtracking(self, nums,usage_list):
        # Base Case本题求叶子节点
        if len(self.path) == len(nums):
            if self.path not in self.paths:
                self.paths.append(self.path)

        # 单层递归逻辑
        for i in range(0, len(nums)):  # 从头开始搜索
            # 若遇到self.path里已收录的元素，跳过
            if usage_list[i] == True:
                continue
            usage_list[i] = True
            self.path=self.path+nums[i]
            if self.path not in self.paths:
                self.paths.append(self.path)

            self.backtracking(nums, usage_list)     # 纵向传递使用信息，去重
            self.path=self.path[:-1]
            usage_list[i] = False