LeetCode 28: Find the Index of the First Occurrence in a String Solution

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28. Find the Index of the First Occurrence in a String

Easy

Two Pointers String String Matching

Problem Explanation

Explanation:

To find the index of the first occurrence of the needle string in the haystack string, we can iterate through the haystack string character by character. At each position, we check if the substring starting from that position matches the needle string. If a match is found, we return the current index as the result. If no match is found, we continue to the next character in the haystack string. If we reach the end of the haystack string without finding a match, we return -1.

Algorithm:
1. Iterate through the haystack string character by character.
2. Check if the substring starting from the current position matches the needle string.
3. If a match is found, return the current index.
4. If no match is found, continue to the next character.
5. If no match is found after iterating through the entire haystack string, return -1.
Time Complexity: O((m-n)*n) where m is the length of haystack and n is the length of needle.
Space Complexity: O(1)

Solution Code

class Solution {
    public int strStr(String haystack, String needle) {
        if (needle.isEmpty()) {
            return 0;
        }
        
        for (int i = 0; i <= haystack.length() - needle.length(); i++) {
            if (haystack.substring(i, i + needle.length()).equals(needle)) {
                return i;
            }
        }
        
        return -1;
    }
}

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