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.

Difficulty: MEDIUM
Tags: String algorithms

Starter code:
class Solution:
    def longestPrefix(self, s: str) -> str:
        

Solutions:
class Solution:

	def longestPrefix(self, strn: str) -> str:
		max_prefs = [0] * len(strn)
		curr = 0
		for idx in range(1, len(strn)):
			while True:
				if curr == 0:
					if strn[idx] == strn[0]:
						curr = 1
					max_prefs[idx] = curr
					break
				elif strn[idx] == strn[curr]:
					curr += 1
					max_prefs[idx] = curr
					break
				else:
					curr = max_prefs[curr - 1]
		return strn[:max_prefs[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		lps = [0] * n
		j = 0
		for i in range(1, n):
			if s[i] == s[j]:
				j += 1
				lps[i] = j
			else:
				while j > 0:
					j = lps[j - 1]
					if s[j] == s[i]:
						j += 1
						break
				lps[i] = j
		return s[:lps[-1]]


from collections import defaultdict

class TrieNode:

	def __init__(self, x=None):
		self.val = x
		self.end = False
		self.children = defaultdict(TrieNode)

class Trie:

	def __init__(self):
		self.root = TrieNode()
		self.max_len = 0

	def insert(self, word):
		node = self.root
		for (i, w) in enumerate(word):
			node.children[w].val = w
			node = node.children[w]
			self.root.children[w] = node
			if i == len(w) - 1:
				node.end = True

	def find(self, word):
		node = self.root
		d = 0
		for w in word[:-1]:
			print(w, node.children.keys())
			node = node.children[w]
			d += 1
			if node.end:
				print(node.val, node.end)
				self.max_len = max(self.max_len, d)

class Solution:

	def longestPrefix(self, s):
		(res, l, r, mod) = (0, 0, 0, 10 ** 9 + 7)
		for i in range(len(s) - 1):
			l = (l * 128 + ord(s[i])) % mod
			r = (r + pow(128, i, mod) * ord(s[~i])) % mod
			if l == r:
				res = i + 1
		return s[:res]


class Solution:

	def longestPrefix(self, s):
		(res, l, r, mod) = (0, 0, 0, 10 ** 9 + 7)
		for i in range(len(s) - 1):
			l = (l * 128 + ord(s[i])) % mod
			r = (r + pow(128, i, mod) * ord(s[~i])) % mod
			if l == r:
				res = i + 1
		return s[:res]


class Solution:

	def longestPrefix(self, s: str) -> str:
		best = ''
		left = 0
		right = len(s) - 1
		left_s = ''
		right_s = ''
		hash_left = 0
		hash_right = 0
		while left < len(s) - 1 and right > 0:
			hash_left += ord(s[left])
			hash_right += ord(s[right])
			left_s += s[left]
			right_s = s[right] + right_s
			if hash_left == hash_right:
				if left_s == right_s:
					best = left_s
			left += 1
			right -= 1
		return best


class Solution:

	def longestPrefix(self, s: str) -> str:
		ans = ''
		(l, r) = (0, 0)
		mod = 10 ** 9 + 7
		for i in range(len(s) - 1):
			l = (l * 31 + ord(s[i])) % mod
			r = (r + ord(s[len(s) - i - 1]) * pow(31, i, mod)) % mod
			if l == r:
				ans = s[:i + 1]
		return ans


class Solution:

	def longestPrefix(self, st: str) -> str:
		a = st
		l = len(a)
		leng = 0
		lps = [0]
		i = 1
		while i < l:
			if a[i] == a[leng]:
				leng += 1
				lps.append(leng)
				i += 1
			elif leng > 0:
				leng = lps[leng - 1]
			else:
				lps.append(0)
				i += 1
		return st[:lps[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		ls = len(s)
		id = -1
		for i in range(ls - 1):
			if s[0:i + 1] == s[ls - i - 1:ls]:
				id = i
		return s[:id + 1]


class Solution:

	def longestPrefix(self, s: str) -> str:
		ans = 0
		for l in range(len(s)):
			if s[:l] == s[-l:]:
				ans = l
		return s[:ans]


class Solution:

	def longestPrefix(self, s: str) -> str:
		max_index = -1
		for (index, _) in enumerate(s[:-1]):
			if s[:index + 1] == s[-1 * (index + 1):]:
				max_index = index
		return s[:max_index + 1]


class Solution:

	def longestPrefix(self, s: str) -> str:
		(i, j, l) = (0, 1, len(s))
		m = 0
		while i < l - 1:
			if s[0:i + 1] == s[l - i - 1:]:
				m = i + 1
			i += 1
		if m == l - 1:
			return s[:l - 1]
		else:
			return s[0:m]


class Solution:

	def longestPrefix(self, s: str) -> str:
		solution = ''
		for i in range(len(s)):
			prefix = s[0:i]
			suffix = s[-len(prefix):]
			if prefix == suffix:
				solution = prefix
		return solution


class Solution:

	def longestPrefix(self, s: str) -> str:
		if len(s) < 2:
			return ''
		length = len(s)
		res = ''
		for i in range(len(s) - 1):
			if s[:i + 1] == s[length - i - 1:]:
				res = s[:i + 1]
		return res


class Solution:

	def longestPrefix(self, s: str) -> str:
		answer = ''
		for n in range(1, len(s)):
			prefix = s[:n]
			suffix = s[-n:]
			if prefix == suffix:
				answer = prefix
		return answer


class Solution:

	def longestPrefix(self, s: str) -> str:
		long = 0
		for x in range(0, len(s) - 1):
			if s[0:x + 1] == s[len(s) - 1 - x:]:
				long = max(long, len(s[0:x + 1]))
		return s[:long]


class Solution:

	def longestPrefix(self, s: str) -> str:
		res = ''
		(i, j) = (0, len(s) - 1)
		while i < len(s) - 1:
			if s[:i + 1] == s[j:]:
				res = s[:i + 1]
			i += 1
			j -= 1
		return res


class Solution:

	def longestPrefix(self, s: str) -> str:
		lps = [0] * len(s)
		j = 0
		for i in range(1, len(s)):
			while j > 0 and s[j] != s[i]:
				j = lps[j - 1]
			if s[j] == s[i]:
				lps[i] = j + 1
				j += 1
		return s[:lps[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		i = 0
		string = str()
		ans = 0
		j = len(s) - 1
		while i < len(s) - 1 and j > 0:
			if s[0:i + 1] == s[j:len(s)]:
				if i + 1 > ans:
					ans = i + 1
					string = s[0:i + 1]
			i = i + 1
			j = j - 1
		return string


class Solution:

	def longestPrefix(self, s: str) -> str:
		ans = ''
		for i in range(len(s)):
			if s[:i] == s[-i:] and len(s[:i]) >= len(ans):
				ans = s[:i]
		return ans


class Solution:

	def longestPrefix(self, s: str) -> str:
		result = ''
		left = 0
		right = len(s) - 1
		hash_left = 0
		hash_right = 0
		while left < len(s) - 1 and right > 0:
			if s[:left + 1] == s[right:]:
				result = s[right:]
			left += 1
			right -= 1
		return result


class Solution:

	def longestPrefix(self, s: str) -> str:
		i = 1
		ans = ''
		while i < len(s):
			if s[0:i] == s[len(s) - i:]:
				ans = s[0:i]
			i += 1
		return ans


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		i = 0
		maxi = ''
		while i < n:
			print()
			if s[:i] == s[n - i:]:
				maxi = max(maxi, s[:i])
			i += 1
		print('maxi', maxi)
		return maxi


class Solution:

	def longestPrefix(self, s: str) -> str:
		best = ''
		for i in range(1, len(s)):
			pattern = s[:i]
			if s.startswith(pattern) and s.endswith(pattern):
				best = pattern
		return best


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(list(s))
		for i in range(0, n):
			if s[0:i] == s[n - i:n]:
				result = s[0:i]
		return result


class KMP:

	def Build(self, p):
		m = len(p)
		nxt = [0, 0]
		j = 0
		for i in range(1, m):
			while j > 0 and p[i] != p[j]:
				j = nxt[j]
			if p[i] == p[j]:
				j += 1
			nxt.append(j)
		return nxt

	def Match(self, s, p):
		n = len(s)
		m = len(p)
		nxt = self.Build(p)
		ans = []
		j = 0
		for i in range(n):
			while j > 0 and s[i] != p[j]:
				j = nxt[j]
			if s[i] == p[j]:
				j += 1
			if j == m:
				ans.append(i - m + 1)
				j = nxt[j]
		return ans

class Solution:

	def longestPrefix(self, s: str) -> str:
		kmp = KMP()
		nxt = kmp.Build(s)
		return s[:nxt[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		len_s = len(s)
		nxts = [0] * len_s
		(i, j) = (1, 0)
		while i < len_s:
			while s[j] != s[i] and j > 0:
				j = nxts[j - 1]
			if s[j] == s[i]:
				j = j + 1
				nxts[i] = j
			i = i + 1
		return s[len_s - nxts[-1]:]


class Solution:

	def longestPrefix(self, s: str) -> str:
		pre = self.get_pre(s)
		return s[0:pre[-1]]

	def get_pre(self, pattern):
		pre = [0]
		n = len(pattern)
		j = 0
		for i in range(1, n):
			while j and pattern[j] != pattern[i]:
				j = pre[j - 1]
			if pattern[i] == pattern[j]:
				j += 1
			pre.append(j)
		return pre


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		arr = [0 for i in range(n)]
		i = 1
		j = 0
		try:
			while i < n:
				if j == 0 and s[i] != s[j]:
					arr[i] = 0
					i += 1
				elif s[i] == s[j]:
					arr[i] = j + 1
					i += 1
					j += 1
				else:
					j = arr[j - 1]
			return s[:arr[n - 1]]
		except:
			return i


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:
		l = len(s)
		nx = [-1] * (l + 1)
		(i, j) = (0, -1)
		while i < l:
			while j == -1 or (i < l and s[i] == s[j]):
				i += 1
				j += 1
				nx[i] = j
			if i < l and s[i] != s[j]:
				j = nx[j]
		return s[:nx[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		(i, j) = (0, -1)
		t = [0 for _ in range(len(s) + 1)]
		t[0] = -1
		while i < len(s):
			if j == -1 or s[i] == s[j]:
				i += 1
				j += 1
				t[i] = j
			else:
				j = t[j]
		return s[:t[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:

		def build():
			hashtable = [0 for _ in s]
			for i in range(1, len(s)):
				j = hashtable[i - 1]
				while j > 0 and s[i] != s[j]:
					j = hashtable[j - 1]
				if s[i] == s[j]:
					hashtable[i] = j + 1
			return hashtable
		table = build()
		return s[len(s) - table[-1]:]


class Solution:

	def longestPrefix(self, s: str) -> str:
		par = [0] * len(s)
		for i in range(1, len(s)):
			j = par[i - 1]
			while j and s[j] != s[i]:
				j = par[j - 1]
			j += int(s[i] == s[j])
			par[i] = j
		return s[:par[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		lps = [0]
		for i in range(1, len(s)):
			pos = lps[-1]
			if s[pos] == s[i]:
				lps.append(1 + lps[-1])
			else:
				pos = lps[lps[-1] - 1]
				if s[pos] == s[i]:
					lps.append(1 + pos)
				else:
					lps.append(0)
		return s[:lps[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		LPS = [0] * n
		i = 1
		longestprefix = 0
		while i < n:
			if s[longestprefix] == s[i]:
				longestprefix += 1
				LPS[i] = longestprefix
				i += 1
			elif longestprefix > 0:
				longestprefix = LPS[longestprefix - 1]
			else:
				i += 1
		return s[:LPS[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		lps = [0] * len(s)
		i = 1
		l = 0
		while i < len(s):
			if s[i] == s[l]:
				l = l + 1
				lps[i] = l
				i = i + 1
			elif l:
				l = lps[l - 1]
			else:
				lps[i] = 0
				i = i + 1
		m = lps[-1]
		index = len(s) - 1
		return s[abs(m - index - 1):index + 1]


class Solution:

	def longestPrefix(self, s: str) -> str:
		t = [-1]
		(pos, cnd) = (1, 0)
		while pos < len(s) - 1:
			if s[pos] == s[cnd]:
				t.append(t[cnd])
			else:
				t.append(cnd)
				cnd = t[cnd]
				while cnd >= 0 and s[pos] != s[cnd]:
					cnd = t[cnd]
			pos += 1
			cnd += 1
		t.append(cnd)
		(j, k) = (1, 0)
		while j < len(s):
			if s[k] == s[j]:
				j += 1
				k += 1
				if k == len(s):
					k = t[k]
			else:
				k = t[k]
				if k < 0:
					j += 1
					k += 1
		return s[:k]


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		(prefix, suffix) = (0, 0)
		(base, mod, mul) = (31, 1000000007, 1)
		happy = 0
		for i in range(1, n):
			prefix = (prefix * base + (ord(s[i - 1]) - 97)) % mod
			suffix = (suffix + (ord(s[n - i]) - 97) * mul) % mod
			if prefix == suffix:
				happy = i
			mul = mul * base % mod
		return s[:happy]


MOD = 100000007

class Solution:

	def longestPrefix(self, s: str) -> str:
		r = 0
		L = R = 0
		multi = 1
		for i in range(len(s) - 1):
			L = (26 * L + ord(s[i]) - ord('a')) % MOD
			R = ((ord(s[~i]) - ord('a')) * multi + R) % MOD
			multi = multi * 26 % MOD
			if L == R:
				r = i + 1
		return s[:r]


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		if n < 2:
			return ''
		i = 1
		j = 0
		f = [0 for _ in range(n)]
		while i < n:
			if s[i] == s[j]:
				j += 1
				f[i] = j
				i += 1
			elif j > 0:
				j = f[j - 1]
			else:
				i += 1
		return s[:f[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		for i in range(1, n):
			if s.startswith(s[i:]):
				return s[i:]
		return ''

class Solution2:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		dp = [[0] * n for _ in range(n)]
		for i in range(n - 1):
			for j in range(i + 1, n):
				if s[i] == s[j]:
					if i == 0:
						dp[i][j] = 1
					elif dp[i - 1][j - 1]:
						dp[i][j] = dp[i - 1][j - 1] + 1
		max_len = 0
		for i in range(n):
			max_len = max(max_len, dp[i][n - 1])
		return s[-max_len:] if max_len else ''


class Solution:

	def longestPrefix(self, s: str) -> str:
		M = 19134702400093278081449423917
		(n, h, t) = (len(s), 0, 0)
		v = list(map(lambda x: ord(x) - ord('a'), s))
		(l, p) = (0, 1)
		for i in range(n - 1):
			h = (h * 26 + v[i]) % M
			t = (v[n - 1 - i] * p + t) % M
			p = p * 26 % M
			if h == t:
				l = i + 1
		return s[:l]


class Solution:
	MOD = 10 ** 9 + 7

	def longestPrefix(self, s: str) -> str:
		N = len(s)
		(left_i, right_i) = (0, N - 1)
		left_val = right_val = 0
		best_left = -1
		right_multiplier = 1
		for i in range(N - 1):
			(left_char, right_char) = (s[i], s[N - i - 1])
			left_val = left_val * 26 + (ord(left_char) - ord('a'))
			right_val = (ord(right_char) - ord('a')) * right_multiplier + right_val
			right_multiplier *= 26
			left_val %= Solution.MOD
			right_val %= Solution.MOD
			right_multiplier %= Solution.MOD
			if left_val == right_val:
				best_left = i
		return s[:best_left + 1]


class Solution:

	def longestPrefix(self, s: str) -> str:
		if len(s) == 1:
			return ''

		def KMPSearch(pat, txt):
			(M, N) = (len(pat), len(txt))
			lps = failure(pat)
			i = 0
			j = 0
			while i < N:
				if pat[j] == txt[i]:
					i += 1
					j += 1
				elif i < N and pat[j] != txt[i]:
					if j != 0:
						j = lps[j - 1]
					else:
						i += 1
				if i >= N:
					return i - j

		def failure(pat):
			res = [0]
			(i, target) = (1, 0)
			while i < len(pat):
				if pat[i] == pat[target]:
					target += 1
					res += (target,)
					i += 1
				elif target:
					target = res[target - 1]
				else:
					res += (0,)
					i += 1
			return res
		l = len(s) - 1 - KMPSearch(s[:-1], s[1:])
		return s[:l]


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		if n <= 1:
			return ''
		(prefix, suffix, base) = (0, 0, 1)
		NUM = 10 ** 9 + 7
		ans = ''
		for i in range(1, n):
			x = ord(s[i - 1]) - ord('a')
			prefix = (26 * prefix + x) % NUM
			x = ord(s[n - i]) - ord('a')
			suffix = (suffix + x * base) % NUM
			base = base * 26 % NUM
			if prefix == suffix:
				if s[0:i] == s[n - i:n]:
					ans = s[0:i]
		return ans


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		prefix = [0] * n
		(j, i) = (0, 1)
		while i < n:
			if s[j] == s[i]:
				j += 1
				prefix[i] = j
			elif j > 0:
				j = prefix[j - 1]
				i -= 1
			i += 1
		return s[:j]


class Solution:

	def build(self, p):
		leng = 0
		i = 1
		lps = [0]
		while i < len(p):
			if p[i] == p[leng]:
				i += 1
				leng += 1
				lps.append(leng)
			elif leng:
				leng = lps[leng - 1]
			else:
				lps.append(0)
				i += 1
		return lps

	def longestPrefix2(self, s: str) -> str:
		lps = self.build(s)
		return s[:lps[-1]]

	def longestPrefix(self, s: str) -> str:
		mul = 1
		mod = pow(10, 9) + 7
		i = 0
		j = len(s) - 1
		res = 0
		hash1 = 0
		hash2 = 0
		while i < len(s) - 1 and j > 0:
			hash1 = (hash1 * 26 + ord(s[i]) - ord('a')) % mod
			hash2 = (hash2 + mul * (ord(s[j]) - ord('a'))) % mod
			mul = mul * 26 % mod
			if hash1 == hash2:
				res = i + 1
			i += 1
			j -= 1
		return s[:res]


class Solution:

	def longestPrefix(self, s: str) -> str:
		length = len(s)
		for i in range(1, length):
			if s[i] == s[0] and s[i:] == s[:length - i]:
				return s[i:]
		return ''


class Solution:

	def longestPrefix(self, s: str) -> str:
		for i in range(len(s) - 1, -1, -1):
			if s[:i] == s[-i:]:
				return s[:i]
		return ''


class Solution:

	def longestPrefix(self, s: str) -> str:
		index = 0
		MOD = 10 ** 9 + 7
		mul = 1
		sLen = len(s)
		(prefixHash, suffixHash) = (0, 0)
		j = sLen - 1
		for i in range(sLen - 1):
			first = ord(s[i]) - ord('a')
			last = ord(s[j]) - ord('a')
			prefixHash = (prefixHash * 26 + first) % MOD
			suffixHash = (last * mul + suffixHash) % MOD
			mul = mul * 26 % MOD
			j -= 1
			if prefixHash == suffixHash:
				index = i + 1
		return s[:index]


class Solution:

	def computelps(self, p: str):
		m = len(p)
		lps = [0] * m
		i = 1
		j = 0
		while i < m:
			if p[i] != p[j]:
				if j == 0:
					i += 1
				else:
					j = lps[j - 1]
			else:
				j += 1
				lps[i] = j
				i += 1
		return lps

	def longestPrefix(self, s: str) -> str:
		lps = self.computelps(s)
		return s[:lps[-1]]


class Solution:

	def longestPrefix(self, s: str) -> str:
		n = len(s)
		k = n - 1
		while k > 0:
			if s[:k] == s[-k:]:
				break
			k -= 1
		return s[:k]


class Solution:

	def longestPrefix(self, s: str) -> str:
		sLen = len(s)
		if sLen == 1:
			return ''

		def happyNow(x):
			if s[:x] == s[-x:]:
				return True
			return False
		for length in range(sLen - 1, 0, -1):
			if happyNow(length):
				return s[:length]
		return ''


class Solution:

	def longestPrefix(self, s: str) -> str:
		(j, prefix, suffix) = (0, 0, 0)
		mod = 10 ** 9 + 9
		for i in range(len(s) - 1):
			prefix = (prefix * 128 + ord(s[i])) % mod
			suffix = (suffix + pow(128, i, mod) * ord(s[-i - 1])) % mod
			if prefix == suffix:
				j = i + 1
		return s[:j]