Longest Palindrome Substring

dynamic_programming/longest_palindrome_substring.py

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+"""
+Author  : Anubhav Sharma
+
+This is a pure Python implementation of Dynamic Programming solution to the longest
+palindrome substring of a given string.
+I use Manacher Algorithm which is amazing algorithm and find solution in linear time complexity.
+
+The problem is  :
+Given a string, to find the longest palindrome sub-string in that given string and
+return it.
+Example: aabbabbaababa as input will return
+         aabbabbaa as output
+"""
+def manacher_algo_lps(s,n):
+    """
+    PARAMETER
+    --------------
+    s = string
+    n = string_len (int)
+
+    manacher Algorithm is the fastest technique to find the longest palindrome substring in any given string.
+
+    RETURN
+    ---------------
+    Longest Palindrome String(String)
+    """
+    # variables to use
+    p = [0] * n
+    c = 0
+    r = 0
+    maxlen = 0
+
+    # Main Algorithm
+    for i in range(n):
+        mirror = 2*c-i # Finding the Mirror(i.e. Pivort to break) of the string
+        if i < r:
+            p[i] = (r - i) if (r - i) < p[mirror] else p[mirror]
+        a = i + (1 + p[i])
+        b = i - (1 + p[i])
+
+        # Attempt to expand palindrome centered at currentRightPosition i 
+        # Here for odd positions, we compare characters and 
+        # if match then increment LPS Length by ONE 
+        # If even position, we just increment LPS by ONE without 
+        # any character comparison
+        while a<n and b>=0 and s[a] == s[b]:
+            p[i] += 1
+            a += 1
+            b -= 1
+        if (i + p[i]) > r:
+            c = i
+            r = i + p[i]
+            if p[i] > maxlen:               # Track maxLPSLength
+                maxlen = p[i]
+    i = p.index(maxlen)
+    return s[i-maxlen:maxlen+i][1::2]
+
+def longest_palindrome(s: str) -> str:
+    s = '#'.join(s)
+    s = '#'+s+'#'
+
+    # Calling Manacher Algorithm
+    return manacher_algo_lps(s,len(s))
+
+def main():
+
+    # Input to enter
+    input_string = "abbbacdcaacdca"
+
+    # Calling the longest palindrome algorithm
+    s = longest_palindrome(input_string)
+    print("LPS Using Manacher Algorithm {}".format(s))
+
+# Calling Main Function
+if __name__ == "__main__":
+
+    main()
+
+    import doctest
+    doctest.testmod()
+