28. Implement strStr()

https://leetcode.com/problems/implement-strstr/

Problem

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().

Constraints:

• haystack and needle consist only of lowercase English characters.

Solution

class Solution {
public:
    int strStr(string &haystack, string &needle) {
        if (needle.empty()) return 0;
        const int max_index = static_cast<int>(haystack.size()) - static_cast<int>(needle.size());
        for (int i = 0; i < max_index + 1; ++i) {
            for (int j = 0; j < needle.size(); ++j) {
                if (haystack[i + j] != needle[j]) break;
                if (j == needle.size() - 1) return i;
            }
        }
        return -1;
    }
};

• #string

• #important

• #kmp