Problem Statement
Given two strings A and B, return the 0-based starting index of the first contiguous substring in A that is a permutation (anagram) of B.
If no such substring exists, return -1.
Constraints
1 <= length of B <= length of A <= 100,000AandBconsist entirely of lowercase English letters.
Test Cases
Example 1
Input: A = "cbaebabacd", B = "abc"
Output: 0
Explanation: The substring "cba" starting at index 0 is a permutation of "abc".
This problem asks for the first 0-based index in A where a contiguous substring is an anagram of B, or -1 if none exists. Because the strings can be as long as 100,000 characters, the intended solution is a sliding window with frequency counting over the 26 lowercase letters. Track the character counts of B, slide a fixed-size window of length |B| across A, and check whether the window frequencies match the target. In the example A = "cbaebabacd" and B = "abc", the first window "cba" already matches, so the answer is 0.
