# Implement strStr() Problem

## Description

LeetCode Problem 28.

Implement strStr().

Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Clarification:

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:

``````1
2
Input: haystack = "hello", needle = "ll"
Output: 2
``````

Example 2:

``````1
2
Input: haystack = "aaaaa", needle = "bba"
Output: -1
``````

Example 3:

``````1
2
Input: haystack = "", needle = ""
Output: 0
``````

Constraints:

• 0 <= haystack.length, needle.length <= 5 * 10^4
• haystack and needle consist of only lower-case English characters.

## Sample C++ Code

``````1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
class Solution {
public:
int strStr(string haystack, string needle) {
int l1 = haystack.size(), l2 = needle.size();
int i = 0, j = 0, k = 0;
if (l2 == 0)
return 0;
if (l1 == 0)
return -1;
bool isSub = true;
while (i < l1 - l2 + 1) {
if (haystack[i] == needle) {
isSub = true;
for (k = 0; k < l2; k ++) {
if (haystack[i + k] != needle[k]) {
isSub = false;
break;
}
}
if (isSub)
return i;
}
i ++;
}
return -1;
}
};
``````