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.
class Solution:
    def longestPrefix(self, s: str) -> str:
        def build(p):
            m = len(p)
            nxt = [0,0]
            j = 0
            for i in range(1,m):
                while j > 0 and s[i] != s[j]:
                    j = nxt[j]
                if s[i] == s[j]:
                    j += 1
                nxt.append(j)
            return nxt
        
        nxt = build(s)
        return s[:nxt[-1]]
class Solution:
    def longestPrefix(self, s: str) -> str:
        def build(p):
            m = len(p)
            nxt = [0,0]
            j = 0
            for i in range(1,m):
                while j > 0 and s[i] != s[j]:
                    j = nxt[j]
                if s[i] == s[j]:
                    j += 1
                nxt.append(j)
            return nxt
        
        nxt = build(s)
        return s[:nxt[-1]]