Finding the Needle in the Haystack

In this article, we'll explore a classic problem in string manipulation: finding the index of the first occurrence of a substring (often called the "needle") within another string (the "haystack"). This problem is common in coding challenges and interviews, and mastering it will help sharpen your string handling skills.

The Problem

You're given two strings: needle and haystack. The task is to find the index of the first occurrence of needle in haystack. If needle isn't part of haystack, return -1.

Let's illustrate this with some examples:

• Example 1:Input: haystack = "sadbutsad", needle = "sad",Output: 0, Explanation: The substring "sad" appears first at index 0.
• Example 2: Input: haystack = "leetcode", needle = "leeto", Output: -1, Explanation: The substring "leeto" is not found in "leetcode", so the output is -1.

Constraints

• The length of haystack and needle can be up to 10,000 characters.
• Both strings consist only of lowercase English letters.

Understanding the Solution

The most direct way to approach this problem is to use a sliding window to extract substrings of the same length as the needle and compare them to the needle itself. If the substring matches the needle, we return the current index. If no match is found after checking all possible starting positions, we return -1.

The Code

class Solution:

def strStr(self, haystack: str, needle: str) -> int:

haystack_length = len(haystack)

needle_length = len(needle)

# If the needle is longer than the haystack, it can't be found

if needle_length > haystack_length:

return -1

# Traverse the haystack using a sliding window

i = 0

while i <= (haystack_length - needle_length):

# Extract substring and compare with the needle

if haystack[i:i + needle_length] == needle:

return i

i += 1

# If no match is found, return -1

return -1

Here's the step-by-step breakdown of our approach:

1. Check String Lengths: If the needle is longer than the haystack, there's no way the needle can be found, so we return -1 immediately.
2. Sliding Window: We then loop through the haystack, taking substrings of the same length as the needle from the current position. For each starting position i, we extract the substring haystack[i:i + needle_length] and check if it matches the needle. If a match is found, we return the index i.
3. Return -1 if Not Found: If no match is found by the time we've checked all valid starting positions, we return -1.

Explaining the Key Part: haystack[i:i + needle_length]

This is where the slicing magic happens. In each iteration of the loop, we use haystack[i:i + needle_length] to extract a substring of the same length as the needle. For instance, if the haystack = "sadbutsad" and needle = "sad", on the first iteration where i = 0, this slice becomes haystack[0:3] which gives "sad". This is compared with the needle, and if they match, we return the index.

Edge Cases

1. Empty needle: If the needle is an empty string, it is conventionally treated as being found at the beginning of the haystack, so the function should return 0.
2. needle longer than haystack: In this case, we know it can't possibly match, so we return -1 immediately.
3. Multiple occurrences: If the needle occurs multiple times in the haystack, we return the index of its first occurrence.

Conclusion

This problem highlights the importance of understanding slicing and windowed iteration over strings in Python. It's a simple yet effective way to solve this problem with linear time complexity, making it suitable even for long strings.

By mastering this problem, you'll gain confidence in handling more complex string manipulation challenges. Keep practicing, and remember that sometimes the most straightforward approach is also the most efficient.