28.Implement strStr()

Tags: Easy String Two Pointers

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

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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().

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class Solution {
public:
    int strStr(string haystack, string needle) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        if (needle.size() == 0) return 0;

        int n = needle.size();
        for (int i = 0; i < haystack.size(); ++i) {
            if (haystack[i] == needle[0]) {
                if (haystack.substr(i, n) == needle)
                    return i;
            }
        }

        return -1;
    }
};