# 28. Implement strStr()

Implement strStr().

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

Example 1:

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

Example 2:

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

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().

# Solution

Approach 1: Scan the haystack to match the needle.

# Code (Python)

Approach 1:

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        if not needle:
            return 0
        for i in range(len(haystack) - len(needle) + 1):
            found = True
            for j in range(len(needle)):
                if haystack[i+j] != needle[j]:
                    found = False
                    break
            if found:
                return i
        return -1

# Code (C++)

Approach 1:

class Solution {
public:
    int strStr(string haystack, string needle) {
        if (needle.size() == 0) return 0;
        for (int i = 0; i <= (int)haystack.size() - (int)needle.size(); ++i) // size_t is an unsigned integer.
        {
            int j = 0;
            for (; j < needle.size(); ++j)
            {
                if (haystack[i+j] != needle[j])
                    break;
            }
            if (j == needle.size())
                return i;
        }
        return -1;
    }
};