Prefix and Suffix Search

10 Prerequisite LeetCode Problems

“745. Prefix and Suffix Search” involves Trie, string manipulation, and data structures. Here are 10 problems to understand these concepts:

  1. Problem 208. Implement Trie (Prefix Tree) This problem is about implementing a trie, which is a crucial concept for the main problem.

  2. Problem 211. Design Add and Search Words Data Structure It will help in understanding how to use trie to support search operation with ‘.’ which could replace any letter.

  3. Problem 212. Word Search II This problem is an application of Trie. Solving this would enhance understanding of using trie for string problems.

  4. Problem 139. Word Break This problem is related to string manipulation and dynamic programming. It helps with understanding how to handle string data in different ways.

  5. Problem 79. Word Search This problem will help you understand how to deal with matrix and string search. It introduces the backtracking approach to solve such problems.

  6. Problem 14. Longest Common Prefix This is a simpler problem that involves finding a common prefix in a set of words.

  7. Problem 720. Longest Word in Dictionary This problem will help you understand how to search in a Trie for the longest word.

  8. Problem 1048. Longest String Chain This problem is about string manipulation and dynamic programming. It’s slightly more complex, but it will help in understanding how to handle string data and create solutions.

  9. Problem 76. Minimum Window Substring This problem involves string manipulation and sliding window technique, which can be useful in string-related problems.

  10. Problem 5. Longest Palindromic Substring This problem requires finding a substring in a string which involves string manipulation and dynamic programming.

These cover Trie data structure, string manipulation, and other relevant concepts, which are crucial to solve problem 745. Prefix and Suffix Search. First understand how Trie works, how to handle strings, and how to manipulate data in data structures.

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from typing import List

class WordFilter:
    def __init__(self, words: List[str]):
        self.prefix_suffix_index = {}
        for index, word in enumerate(words):
            word_length = len(word)
            for i in range(word_length + 1):
                for j in range(word_length + 1):
                    self.prefix_suffix_index[word[:i] + '#' + word[j:]] = index

    def f(self, pref: str, suff: str) -> int:
        return self.prefix_suffix_index.get(pref + '#' + suff, -1)

Problem Classification

This problem falls under the string manipulation. The task involves designing and implementing a special kind of dictionary that allows searching words by their prefix and suffix.

‘What’ Components:

  1. Input: The input consists of an array of words that make up the dictionary. Each word is a string consisting of lowercase English letters. The number of words is at least 1 and at most 10^4, and each word’s length is between 1 and 7.

  2. Operations:

    • Initialization: A special dictionary has to be initialized with the given list of words.
    • Querying: The dictionary has to provide a method to return the index of the word that has a specific prefix and suffix. If multiple words match the criteria, the one with the highest index should be returned. If no word matches, -1 is returned.

  3. Output: For each query, the dictionary has to return an integer representing the index of the word with the requested prefix and suffix.

This problem can be further classified as a design problem. It involves designing a data structure (a specialized dictionary) with specific properties and methods. The solution would require a good understanding of data structures, especially those that can efficiently handle string-related operations such as prefix and suffix searches. These could include tries, hash tables, or even specialized data structures like suffix trees.

Thought Process

This requires us to design a data structure that can efficiently handle queries for words with specific prefixes and suffixes. Here are the steps we need to follow:

  1. Understand the problem statement: We need to initialize a special dictionary with a list of words and later query for words that have a specific prefix and a suffix.

  2. Consider data structures that can help: Given the requirements for prefix and suffix queries, a trie (a type of search tree used for storing strings where each node stores a common prefix) might come to mind. However, since we also need to handle suffix queries, we might need to consider an alternate data structure or strategy. One such strategy is to generate all possible prefix-suffix pairs for each word and store them in a dictionary or hash map.

  3. Generate prefix-suffix pairs: For each word in the initial list, generate all possible combinations of prefix and suffix pairs.

  4. Create a mapping: Create a hash map where each key is a prefix ‘#’ suffix pair, and the value is the index of the word in the list.

  5. Handle queries: For each query, create the prefix-suffix key and look it up in the hash map. If it exists, return the index; otherwise, return -1.

Now, let’s code this solution.

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from typing import List

class WordFilter:

    def __init__(self, words: List[str]):
        self.prefix_suffix_index = {}
        for index, word in enumerate(words):
            word_length = len(word)
            for i in range(word_length + 1):
                for j in range(word_length + 1):
                    self.prefix_suffix_index[word[:i] + '#' + word[j:]] = index

    def f(self, pref: str, suff: str) -> int:
        return self.prefix_suffix_index.get(pref + '#' + suff, -1)

In this solution, WordFilter constructor takes a list of words and generates all prefix-suffix pairs, storing them in a dictionary along with their respective indices. The f method performs the queries, returning the index of the word that matches the prefix and suffix, or -1 if no such word exists.

This approach has a high space complexity due to storing all possible prefix-suffix pairs, but it provides quick lookup times for queries, which is useful if we have a high volume of queries relative to the size of the word list.

Language Agnostic Coding Drills

  1. Understanding and using basic data types: The code uses string and dictionary data types.

  2. Defining a class: The code is organized in a class, which is an essential concept of object-oriented programming. It’s a way of encapsulating related functions and data.

  3. Defining methods in a class: Within the class, we have the __init__ constructor method for initializing class instances and the f method for processing.

  4. Enumeration: The enumerate function is used to iterate over a list along with an index.

  5. String slicing and concatenation: The code uses slicing to create substrings and concatenation to combine strings.

  6. Loops: The code uses nested loops to create a dictionary of all possible prefix and suffix combinations in the words list.

  7. Hash Maps or Dictionaries: A dictionary is used to store and retrieve the index of words based on their prefix and suffix.

  8. Handling edge cases with dict.get: The get method is used to return the index of a word from the dictionary. If no such word exists, it returns -1.

These concepts are listed in increasing difficulty, starting from understanding basic data types and gradually moving up to defining classes and methods, handling iterations and using advanced data structures like dictionaries. They become more complex as we move from the understanding of simple types and operations to using them in more complex data structures and algorithms.

The problem-solving approach is essentially about finding the word in the dictionary that matches both a given prefix and a suffix. The initial step is to create a comprehensive dictionary mapping all possible prefix and suffix combinations to the index of the word in the original list. This is done in the constructor method. The ‘f’ method is then used to retrieve the index of a word based on the provided prefix and suffix.

To assemble these units into a final solution, first, understand the problem statement and what is being asked. Identify the need for a class with methods to initialize the class instance with a list of words and another method to search for the index of a word by prefix and suffix. Then, use string manipulation techniques, loops, and dictionaries to create the prefix-suffix-to-index mapping in the constructor method. Finally, use the get method to retrieve the desired index in the ‘f’ method.

Targeted Drills in Python

  1. Basic Data Types
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# String
my_string = "Hello, World!"
print(my_string)

# Dictionary
my_dict = {"apple": 1, "banana": 2}
print(my_dict)
  1. Defining a Class
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class MyClass:
    def say_hello(self):
        return "Hello, World!"

obj = MyClass()
print(obj.say_hello())
  1. Defining Methods in a Class
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class MyClass:
    def __init__(self, name):
        self.name = name

    def say_hello(self):
        return f"Hello, {self.name}!"

obj = MyClass("Alice")
print(obj.say_hello())
  1. Enumeration
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for i, char in enumerate("Hello"):
    print(i, char)
  1. String Slicing and Concatenation
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word = "apple"
prefix = word[:2]
suffix = word[3:]
print("Prefix:", prefix)
print("Suffix:", suffix)
print("Concatenation:", prefix + "#" + suffix)
  1. Loops
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# Nested loops
for i in range(3):
    for j in range(3):
        print(i, j)
  1. Hash Maps or Dictionaries
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# Dictionary operation
my_dict = {}
my_dict["a#e"] = 0
print(my_dict)
print(my_dict.get("a#e", -1))
print(my_dict.get("b#e", -1))
  1. Handling Edge Cases with Dict.get
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my_dict = {"a#e": 0}
print(my_dict.get("a#e", -1))  # Return 0
print(my_dict.get("b#e", -1))  # Return -1 as "b#e" does not exist in my_dict

Problem-Specific Concepts:

  • A key concept for our problem is handling prefixes and suffixes. This is crucial because our solution revolves around efficiently retrieving the index of a word given a prefix and suffix.
  • Another important concept is using the dictionary’s get method to handle cases where no word matches the given prefix and suffix. This is a common technique for dealing with edge cases in a dictionary-based look-up system.

Finally, to assemble these drills into the final solution, begin by defining a class with the required methods. In the __init__ method, initialize your dictionary and fill it with entries for each possible prefix-suffix pair, along with their corresponding index. In the f method, utilize the dictionary’s get method to look up and return the appropriate index based on the input prefix and suffix. By integrating these drills together, you should be able to construct the final solution to the problem.

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