Find the Index of the First Occurrence in a String

The task is to find the first occurrence of the string needle in the string haystack. If needle is not part of haystack, you should return -1. Here’s a straightforward approach:

  1. If the needle is an empty string, return 0.
  2. If the length of needle is greater than the length of haystack, return -1.
  3. Iterate through the haystack from index 0 to len(haystack) - len(needle).
  4. For each index, check if the substring of haystack starting from that index and having the length of needle is equal to needle.
  5. If the substring matches needle, return the current index.
  6. If no match is found, return -1.

Here’s the code implementing the above logic:

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class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        # If needle is empty, return 0
        if not needle:
            return 0

        # If needle is longer than haystack, return -1
        if len(needle) > len(haystack):
            return -1

        # Iterate through haystack and find the first occurrence of needle
        for i in range(len(haystack) - len(needle) + 1):
            if haystack[i:i + len(needle)] == needle:
                return i

        # If no match is found, return -1
        return -1

This solution runs with a time complexity of (O((n - m) \cdot m)), where (n) is the length of the haystack, and (m) is the length of the needle, and a space complexity of (O(1)), as no additional space is required.

Identifying Problem Isomorphism

“Find the Index of the First Occurrence in a String” can be mapped to “Implement strStr()”.

Both involve string manipulation and looking for the first occurrence of a substring in a given string. In “Find the Index of the First Occurrence in a String”, you are asked to find the index of the first occurrence of a specific substring in a given string.

“Implement strStr()” is the same task, as you have to return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Both problems are quite simple and involve basic string searching algorithms, and they both require returning the index of the first occurrence of one string within another. Hence, these problems are isomorphic with the same level of difficulty.

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def strStr(self, haystack, needle):
	n, h = len(needle), len(haystack)
	hash_n = hash(needle)
	for i in range(h-n+1):
		if hash(haystack[i:i+n]) == hash_n:
			return i
	return -1

Problem Classification

  1. String Manipulation: This problem clearly deals with strings. You are given two strings as input and you need to manipulate and traverse these strings to find the solution.

  2. Pattern Searching: The problem is about searching for a pattern (the needle) in a text (the haystack).

  3. Subsequence/Substring: This problem requires us to find a substring (a contiguous subsequence) in a larger string, which classifies the problem in this category.

  4. Hashing: Hashing is implied in the problem as a potential solution methodology, but it’s not explicitly stated in the problem statement. We are using hashing to speed up the pattern matching process.

Language Agnostic Coding Drills

  1. Understanding basic string operations: Familiarize yourself with basic string operations like string slicing, finding the length of a string, and comparing strings.

  2. Using hash functions: Understand how hash functions work and what their applications are. Hash functions can convert large amounts of data into a small fixed size. In this case, it’s being used for comparison purposes.

  3. Implementing the sliding window technique: The sliding window technique involves creating a ‘window’ on your data and moving that window along your data to find something. In this case, the window is the size of the needle, and it’s moving along the haystack.

  4. Looping through a list/string: You need to be comfortable with writing for loops that iterate over lists or strings. In this case, you’re looping through the haystack.

Problem-solving approach:

  • Start by determining the lengths of your needle and haystack and calculating the hash of your needle.

  • Next, you loop through the haystack from the start to h-n+1 (because that’s as far as you need to go to find the needle).

  • Within the loop, you take a slice of the haystack that’s the same size as your needle, calculate the hash of that slice, and compare it with the hash of the needle.

  • If the hashes match, you’ve found the needle in the haystack, and you can return the current index i.

  • If you’ve gone through the whole haystack and haven’t found the needle, return -1.

Targeted Drills in Python

  1. Understanding basic string operations:
    • String slicing
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s = "hello world"
print(s[0:5]) # Output: hello
  • Finding the length of a string
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s = "hello world"
print(len(s)) # Output: 11
  • Comparing strings
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s1 = "hello"
s2 = "world"
print(s1 == s2) # Output: False
  1. Using hash functions:
    • Hashing a string
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s = "hello world"
print(hash(s)) # Output: <Some hash value>
  1. Implementing the sliding window technique:
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haystack = "hello world"
needle = "world"
for i in range(len(haystack) - len(needle) + 1):
    print(haystack[i:i+len(needle)]) 
# Output: hello, ello , llo w, lo wo, o wor,  worl, world
  1. Looping through a list/string:
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s = "hello world"
for i in range(len(s)):
    print(s[i]) 
# Output: h, e, l, l, o,  , w, o, r, l, d

Problem Specific Drill: Finding a needle in a haystack using a hash and sliding window:

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def strStr(haystack, needle):
    n, h = len(needle), len(haystack)
    hash_n = hash(needle)
    for i in range(h-n+1):
        if hash(haystack[i:i+n]) == hash_n:
            return i
    return -1

print(strStr("hello world", "world")) # Output: 6

This final drill combines all of the smaller drills: It uses string operations (length and slicing), hashing, the sliding window technique, and looping. As you can see, each of the smaller drills builds up the skills required for the final solution.