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哈希表实际应用 - 设计键

程序员文章站 2022-03-26 20:32:34
LeetCode - 探索卡片 - 哈希表 - 四 - 实际应用 - 设计键...

目录

一、设计键

二、字母异位词分组

2.1 题目要求

2.2 解决过程

三、移位字符串分组 (PLUS 会员专享)

四、有效的数独

4.1 题目要求

4.2 解决过程

五、寻找重复的子树

5.1 题目要求

5.2 解决过程

六、设计键 - 总结


哈希表实际应用 - 设计键

一、设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

参考文献：https://leetcode-cn.com/leetbook/read/hash-table/xxez6m/


二、字母异位词分组

2.1 题目要求

哈希表实际应用 - 设计键

2.2 解决过程

个人实现

法一：哈希映射。使用 sorted() 将字符串转换为列表并排序，然后使用 string.join() 函数将已排序字符列表重新转换为字符串。从而，对于各字母异位词都能得到相同的中间形式，从而作为 key 存入字典中，value 对应字母异位词原形式。最后，使用列表推导式输出结果。空间复杂度 O(nk)，时间复杂度 O(nk)。

2020/08/25 - 98.60% (48ms) - 最佳 

class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        hashmap = {}  # dict
        for i in range(len(strs)):
            s = "".join(sorted(strs[i]))  # 字符串排序, 得到中间形式作为 key
            if not hashmap.get(s):
                hashmap[s] = [strs[i]]
            else:
                hashmap[s].append(strs[i])
        
        return [j for j in hashmap.values()]  # 列表推导式

使用 collections.defaultdict 可以简化书写： 

class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        hashmap = collections.defaultdict(list)  # defaultdict
        for i in range(len(strs)):
            s = "".join(sorted(strs[i]))  # 字符串排序, 得到中间形式作为 key
            hashmap[s].append(strs[i])
        
        return list(hashmap.values())  # 强制类型转换

官方实现与说明

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

class Solution(object):
    def groupAnagrams(self, strs):
        ans = collections.defaultdict(list)
        for s in strs:
            ans[tuple(sorted(s))].append(s)
        return list(ans.values())

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

class Solution:
    def groupAnagrams(strs):
        ans = collections.defaultdict(list)
        for s in strs:
            count = [0] * 26
            for c in s:
                count[ord(c) - ord('a')] += 1
            ans[tuple(count)].append(s)
        return list(ans.values())

哈希表实际应用 - 设计键

其他实现与说明

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

## 数学方法果然厉害
class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        prime = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103]
        hashmap = {}
        for s in strs:
            key = 1
            for c in s:
                key *= prime[ord(c) - 97]
            if key in hashmap:
                hashmap[key].append(s)
            else:
                hashmap[key] = [s]        
                
        return list(hashmap.values())

2020/08/25 - 88.11% (56ms)

哈希表实际应用 - 设计键

参考文献

https://leetcode-cn.com/leetbook/read/hash-table/xxo631/

https://leetcode-cn.com/problems/group-anagrams/solution/zi-mu-yi-wei-ci-fen-zu-by-leetcode/

https://leetcode-cn.com/problems/group-anagrams/solution/xiang-xi-tong-su-de-si-lu-fen-xi-duo-jie-fa-by--16/

三、移位字符串分组 (PLUS 会员专享)

四、有效的数独

4.1 题目要求

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

4.2 解决过程

个人实现

法一：哈希映射 + 哈希集合。为数独表的每一行、每一列、每一块 (index∈[0, 8]) 分别构造一个哈希映射，并存储在 hashmap 中。遍历数独表，第 row 行第 col 列的符号为 board[row][col]。对于有效的数字 (非点号) 而言，通过行号 row 从行键表 row_key 取出键 row_key[row]，并于第 row 行的值 —— 哈希集合 hashmap row_key[row] 中检索是否存在重复数字；列号 col 检索列集合同理；块号检索块集合亦复如是。空间复杂度 O(m)，m 为数独表中数字数 (0 <= m <= 81)；循环遍历次数固定为 9 * 9 = 81 次，各项检索操作设为 O(1)，则时间复杂度为 O(1)。

2020/08/27 - 76.13% (52ms) 

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        row_key = ["r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r8"]    # 行键表
        col_key = ["c0", "c1", "c2", "c3", "c4", "c5", "c6", "c7", "c8"]    # 列键表
        #block_key = ["b0", "b1", "b2", "b3", "b4", "b5", "b6", "b7", "b8"] # 块键表
        hashmap = collections.defaultdict(set)  # 辅助哈希映射表
    
        def check_row(row, num):
            """ 检查当前数字 num 是否存在于当前行 row """
            row_set = hashmap[row_key[row]]  # 行集合
            if num in row_set:
                return True
            else:
                row_set.add(num)
                return False
        
        def check_col(col, num):
            """ 检查当前数字 num 是否存在于当前列 col """
            col_set = hashmap[col_key[col]]  # 列集合
            if num in col_set:
                return True
            else:
                col_set.add(num)
                return False     
         
        def check_block(row, col, num):
            """ 检查当前数字 num 是否存在于当前块 block """
            if (0 <= row <= 2) and (0 <= col <= 2):
                bk = "b0"
            elif (0 <= row <= 2) and (3 <= col <= 5):
                bk = "b1"
            elif (0 <= row <= 2) and (6 <= col <= 8):
                bk = "b2"       
            elif (3 <= row <= 5) and (0 <= col <= 2):
                bk = "b3"
            elif (3 <= row <= 5) and (3 <= col <= 5):
                bk = "b4"
            elif (3 <= row <= 5) and (6 <= col <= 8):
                bk = "b5"            
            elif (6 <= row <= 8) and (0 <= col <= 2):
                bk = "b6"      
            elif (6 <= row <= 8) and (3 <= col <= 5):
                bk = "b7"      
            elif (6 <= row <= 8) and (6 <= col <= 8):
                bk = "b8"     
            
            block_set = hashmap[bk]  # 块集合
            if num in block_set:
                return True
            else:
                block_set.add(num)
                return False
            

        for row in range(9):
            for col in range(9):
                num = board[row][col]  # 当前符号
                if (num != ".") and (check_row(row, num) or check_col(col, num) or check_block(row, col, num)):
                    return False
        return True

稍作优化： 

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        row_key = ["r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r8"]    # 行键
        col_key = ["c0", "c1", "c2", "c3", "c4", "c5", "c6", "c7", "c8"]    # 列键
        block_key = ["b0", "b1", "b2", "b3", "b4", "b5", "b6", "b7", "b8"]  # 块键
        hashmap = collections.defaultdict(set)  # 辅助哈希映射表
    
        def check_row(row, num):
            """ 检查当前数字 num 是否存在于当前行 row """
            row_set = hashmap[row_key[row]]  # 行集合
            if num in row_set:
                return True
            row_set.add(num)
            return False
        
        def check_col(col, num):
            """ 检查当前数字 num 是否存在于当前列 col """
            col_set = hashmap[col_key[col]]  # 列集合
            if num in col_set:
                return True
            col_set.add(num)
            return False     
         
        def check_block(row, col, num):
            """ 检查当前数字 num 是否存在于当前块 block """
            # 上层块
            if (0 <= row <= 2):
                if (0 <= col <= 2):              
                    bk = block_key[0]
                elif (3 <= col <= 5):
                    bk = block_key[1]
                elif (6 <= col <= 8):
                    bk = block_key[2]    
            # 中层块
            elif (3 <= row <= 5):
                if (0 <= col <= 2):
                    bk = block_key[3]
                elif (3 <= col <= 5):
                    bk = block_key[4]
                elif (6 <= col <= 8):
                    bk = block_key[5]           
            # 下层块
            elif (6 <= row <= 8):
                if (0 <= col <= 2):
                    bk = block_key[6]        
                elif (3 <= col <= 5):
                    bk = block_key[7]        
                elif (6 <= col <= 8):
                    bk = block_key[8]        
            # 检查存在与否
            block_set = hashmap[bk]  # 块集合
            if num in block_set:
                return True
            block_set.add(num)
            return False
            
        # 遍历数独表各位置
        for row in range(9):
            for col in range(9):
                num = board[row][col]  # 当前
                if (num != ".") and (check_row(row, num) or check_col(col, num) or check_block(row, col, num)):
                    return False
        return True

官方实现与说明

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        # init data
        rows = [{} for i in range(9)]     # 对应 9 行
        columns = [{} for i in range(9)]  # 对应 9 列
        boxes = [{} for i in range(9)]    # 对应 9 块
  
        # validate a board
        for i in range(9):
            for j in range(9):
                num = board[i][j]  # 当前字符
                if num != '.': 
                    num = int(num)  # 当前数字
                    box_index = (i//3)*3 + j//3  ## 块索引转换公式
                    
                    # keep the current cell value
                    rows[i][num] = rows[i].get(num, 0) + 1
                    columns[j][num] = columns[j].get(num, 0) + 1
                    boxes[box_index][num] = boxes[box_index].get(num, 0) + 1
                    
                    # check if this value has been already seen before
                    if rows[i][num] > 1 or columns[j][num] > 1 or boxes[box_index][num] > 1:
                        return False         
        return True

2020/08/27 - 76.13% (52ms)

哈希表实际应用 - 设计键

其他实现与说明

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        row = [[0] * 10 for _ in range(9)]  # 行数组
        col = [[0] * 10 for _ in range(9)]  # 列数组
        box = [[0] * 10 for _ in range(9)]  # 块数组
        
        # 遍历数独表
        for i in range(9):
            for j in range(9):
                # 若为数字
                if board[i][j] != '.':
                    # 当前数字 ASCII 码与 "0" 的 ASCII 码差值作为 index
                    curNum = ord(board[i][j]) - 48  # ord('0')=48
                    k = j//3 + (i//3)*3  # block key
                    # 如果发现已存在
                    if row[i][curNum] or col[j][curNum] or box[k][curNum]:
                        return False
                    # 位置标记
                    row[i][curNum], col[j][curNum], box[k][curNum] = 1, 1, 1
        return True

2020/08/27 - 96.57% (44ms) - 最佳

稍作修改，效率反而低了些： 

class Solution:
    def isValidSudoku(self, board: List[List[str]]) -> bool:
        row = [set() for _ in range(9)]  # 行数组
        col = [set() for _ in range(9)]  # 列数组
        box = [set() for _ in range(9)]  # 块数组
        
        # 遍历数独表
        for i in range(9):
            for j in range(9):
                # 若为数字
                if board[i][j] != '.':
                    # 当前数字 ASCII 码与 "0" 的 ASCII 码差值作为 index
                    curNum = ord(board[i][j]) - 48  # ord('0')=48
                    k = j//3 + (i//3)*3  # block key
                    # 如果发现已存在
                    if (curNum in row[i]) or (curNum in col[j]) or (curNum in box[k]):
                        return False
                    # 位置标记
                    row[i].add(curNum)
                    col[j].add(curNum)
                    box[k].add(curNum)
        return True

2020/08/27 - 89.73% (48ms) - 说明数量较大时，哈希映射 / 哈希集合 的性能会显著下降 (源于各种冲突)

参考文献

https://leetcode-cn.com/leetbook/read/hash-table/xxpit5/

https://leetcode-cn.com/problems/valid-sudoku/solution/you-xiao-de-shu-du-by-leetcode/

https://leetcode-cn.com/problems/valid-sudoku/solution/36-jiu-an-zhao-cong-zuo-wang-you-cong-shang-wang-x/

五、寻找重复的子树

5.1 题目要求

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

5.2 解决过程

测试用例 

[0,null,0,0,null,0,0,null,0,null,0,0]
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个人实现

法一：递归 + 遍历。相当于暴力法了，很直观却也很低效的方式。

2020/08/27 - 5.02% (500ms) 

class Solution:
    def findDuplicateSubtrees(self, root: TreeNode) -> List[TreeNode]:
        # 空树
        if not root:
            return []
        # 线性树 - 单右枝树
        elif not root.left:
            has_left = False
            while root.right:
                root = root.right
                if root.left:
                    has_left = True
                    break
            if not has_left:
                return []
        # 线性树 - 单左枝树
        elif not root.right:
            has_right= False
            while root.left:
                root = root.left
                if root.right:
                    has_right = True
                    break
            if not has_right:
                return []
            
        
        def dfs(root, record):
            """ 通过 DFS 遍历和记录树节点到哈希映射中 """
            if root:
                record[root.val].append(root)
                dfs(root.left, record)
                dfs(root.right, record)
        
        def compare_subtree(root1, root2):
            """ 通过递归比较两树是否完全相同 """
            if (not root1) and (not root2):
                return True
            elif ((not root1) or (not root2)) or (root1.val != root2.val):
                return False
            else:
                return compare_subtree(root1.left, root2.left) and compare_subtree(root1.right, root2.right)
        
        hashmap = collections.defaultdict(list)  
        dfs(root, hashmap)
        
        # 结果列表
        result = []
        # 遍历树节点值
        for key in hashmap.keys():
            nodes = hashmap[key]
            nodes_num = len(nodes)
            # 若值相同的节点不止一个, 则需要进行比较
            if nodes_num > 1:
                # 遍历值相同的节点
                for i in range(nodes_num-1):
                    done = False  # 是否发现过当前重复子树, 默认 False
                    for j in range(i+1, nodes_num):
                        # 若待比较节点非空且完全相同
                        if nodes[i] and nodes[j] and compare_subtree(nodes[i], nodes[j]):
                            # 若此前尚未发现过重复子树
                            if not done:
                                # 则添加相同子树之一到结果列表
                                result.append(nodes[i])
                                # 标记置为 True
                                done = True
                            # 其余相同子树全部置为 None
                            nodes[j] = None
        return result

官方实现与说明

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

## 很妙, 第一次见这种解法！
class Solution:
    def findDuplicateSubtrees(self, root: TreeNode) -> List[TreeNode]:
        def collect(node):
            # 空节点用 "#" 取代
            if not node: 
                return "#"
            # 当前子树的序列化结果
            serial = "{},{},{}".format(node.val, collect(node.left), collect(node.right))
            # 将序列化结果作为键, 使之在哈希计数器中对应的值+1
            count[serial] += 1
            # 若存在重复的序列化结果, 则说明有重复子树
            if count[serial] == 2:
                # 将重复字符的根节点加入结果列表中
                ans.append(node)  
            # 返回当前子树序列化结果
            return serial  
        
        count = collections.Counter()  # 哈希计数器
        ans = []  # 结果列表
        collect(root)  # 递归处理
        return ans

2020/08/27 - 95.75% (68ms) - 次优

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

## 更厉害, 也是首次见的解法！
class Solution:
    def findDuplicateSubtrees(self, root: TreeNode) -> List[TreeNode]:
        def lookup(node):
            if node:
                # 当前子树 id = (node.val, left_child_id, right_child_id)
                uid = trees[node.val, lookup(node.left), lookup(node.right)]
                count[uid] += 1
                if count[uid] == 2:
                    ans.append(node)
                return uid

        trees = collections.defaultdict()  # 默认字典
        # 将默认字典的默认值设为当前字典中元素的个数, 从而每个 key 对应的 value 就会是 0, 1, 2, ... 
        # 如此, UID 将不会有重复
        trees.default_factory = trees.__len__
        count = collections.Counter()  # 哈希计数器
        ans = []  # 结果列表
        lookup(root) 
        return ans

2020/08/27 - 95.72% (68ms) - 最佳

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

参考文献

https://leetcode-cn.com/leetbook/read/hash-table/xxm0i6/

https://leetcode-cn.com/problems/find-duplicate-subtrees/solution/xun-zhao-zhong-fu-de-zi-shu-by-leetcode/

https://leetcode-cn.com/problems/find-duplicate-subtrees/solution/xun-zhao-zhong-fu-de-zi-shu-by-leetcode/275034

六、设计键 - 总结

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

哈希表实际应用 - 设计键

参考文献：https://leetcode-cn.com/leetbook/read/hash-table/xxavl2/

本文地址：https://blog.csdn.net/qq_39478403/article/details/108226382

相关标签： 哈希表 设计键

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