A string is called a happy prefix if is a non-empty prefix which is also a suffix (excluding itself). Given a string s. Return the longest happy prefix of s . Return an empty string if no such prefix exists. Example 1: Input: s = "level" Output: "l" Explanation: s contains 4 prefix excluding itself ("l", "le", "lev", "leve"), and suffix ("l", "el", "vel", "evel"). The largest prefix which is also suffix is given by "l". Example 2: Input: s = "ababab" Output: "abab" Explanation: "abab" is the largest prefix which is also suffix. They can overlap in the original string. Example 3: Input: s = "leetcodeleet" Output: "leet" Example 4: Input: s = "a" Output: "" Constraints: 1 <= s.length <= 10^5 s contains only lowercase English letters.