LongestHappyPrefix.py

"""
Here: https://leetcode.com/problems/longest-happy-prefix/
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 <= 105
s contains only lowercase English letters.

"""


def longestPrefix(s: str) -> str:
    n = len(s)
    for i in range(n - 2, -1, -1):
        prefix = s[:i + 1]
        suffix = s[n - len(prefix):]
        if prefix == suffix:
            return prefix
    return ''


if __name__ == "__main__":
    # taking the input
    s = input("Enter word: ")
    print(longestPrefix(s))

"""
Inputs
s = "level"
Output: "l"

s = "ababab"
Output: "abab"

s = "leetcodeleet"
Output: "leet"
"""