28. Implement strStr()

1. Description

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.

2. Example

Example 1

Input: haystack = “sadbutsad”, needle = “sad”
Output: 0
Explanation: “sad” occurs at index 0 and 6.
The first occurrence is at index 0, so we return 0.

Example 2

Input: haystack = “leetcode”, needle = “leeto”
Output: -1
Explanation: “leeto” did not occur in “leetcode”, so we return -1.

3. Constraints

• 1 <= haystack.length, needle.length <= $10^4$
• haystack and needle consist of only lowercase English characters.

4. Solutions

Two Pointers

m = haystack.size(), n = needle.size()
Time complexity: O(mn)
Space complexity: O(1)

class Solution {
public:
    int strStr(const string &haystack, const string &needle) {
        const int m = haystack.size(), n = needle.size();
        for (int i = 0; i <= m - n; ++i) {
            int j = 0;
            while (j < n && haystack[i + j] == needle[j]) {
                ++j;
            }
            if (j == n) {
                return i;
            }
        }

        return -1;
    }
};

KMP

// TODO