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.
Constraints:
-
1 <= s.length <= 105
-
s
contains only lowercase English letters.
SOLUTION:
class Solution:
def longestPrefix(self, s: str) -> str:
M = len(s)
lps = [0] * M
l = 0
i = 1
while i < M:
if s[i] == s[l]:
l += 1
lps[i] = l
i += 1
else:
if l != 0:
l = lps[l-1]
else:
lps[i] = 0
i += 1
return s[:lps[-1]]
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