41. Implement a function to count the number of vowels in a string using hashing.
42. Create a hash map to map user IDs to their last login times.
43. Write a program to find the maximum length of a substring with at most k distinct characters using hashing.
44. Implement a function to determine if a string has all characters appearing an even number of times using hashing.
45. Create a hash set to keep track of seen elements in a window of an array.
46. Write a program to find pairs of elements with the same sum from two different arrays using hashing.
47. Implement a function to count the number of palindromic substrings in a given string using hashing.
48. Create a hash table to store the ISBN numbers of books and their titles.
49. Write a program to determine if one string can be formed by rearranging the letters of another string using hashing.
50. Implement a function to find the first repeated character in a string using hashing.
51. Create a hash map to store the last occurrence of each character in a string.
52. Write a program to find the smallest window that contains all characters of another string using hashing.
53. Implement a function to find all unique triplets in an array that sum to zero using hashing.
54. Create a hash table to store the grades of students and retrieve them by student ID.
55. Write a program to check if a string is a subsequence of another string using hashing.
56. Implement a function to count the number of distinct words in a sentence using hashing.
57. Create a hash map to store product reviews and retrieve them by product ID.
58. Write a program to find the maximum frequency of any character in a string using hashing.
59. Implement a function to find the two elements in an array that are closest to each other using hashing.
60. Create a hash set to keep track of unique paths in a maze.
61. Write a program to find all pairs of indices that sum to a target using hashing.
62. Implement a function to find the longest substring with all distinct characters using hashing.
63. Create a hash map to track the number of visits to different web pages.
64. Write a program to check if two strings are isomorphic using hashing.
65. Implement a function to find the longest repeating character substring using hashing.
66. Create a hash table to store usernames and their corresponding passwords.
67. Write a program to find the maximum length of a substring that can be formed by replacing at most k characters using hashing.
68. Implement a function to check if a string can be formed by concatenating substrings of another string using hashing.
69. Create a hash map to store the mapping of country codes to country names.
70. Write a program to find the first character that appears three times in a string using hashing.
71. Implement a function to count the number of different ways to decode a message using hashing.
72. Create a hash table to store user preferences and retrieve them by user ID.
73. Write a program to find the common elements between two arrays using hashing.
74. Implement a function to determine if a string can be rearranged to form a palindrome using hashing.
75. Create a hash map to track the inventory of items in a store.
76. Write a program to find the length of the longest substring that contains only vowels using hashing.
77. Implement a function to find all pairs of elements in an array whose product equals a target value using hashing.
78. Create a hash set to keep track of visited states in a game.
79. Write a program to find the longest substring of a string without repeating characters using hashing.
80. Implement a function to find the longest substring with at most two distinct characters using hashing.