Implement strStr() 🚕

Tehleel Mir
1 min readApr 9, 2022



Implement strStr().

Given two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.


What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1


  • 1 <= haystack.length, needle.length <= 104
  • haystack and needle consist of only lowercase English characters.


I don’t know the space and time complexity because I'm using a library function.

Time Complexity O(n*m) where n is the length of the needle and m is the length of the haystack.

Space Complexity O(1)

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