LC 28. Find the Index of the First Occurrence in a String

https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string/description/?envType=study-plan-v2&envId=top-interview-150

class Solution {
public:
    int strStr(string haystack, string needle) {
        int m = haystack.size(), n = needle.size();
        int idx = 0;
        while (idx < m) {
            int start = idx;
            int cur = 0;
            while (idx < m && cur < n && haystack[idx] == needle[cur]) {
                cur += 1;
                idx += 1;
            }
            if (cur == n) return start;
            idx = start + 1;
        }
        return -1;
    }
};

Time Complexity: O(m * n)
Space: O(1)