Implement is_substring_palindrome using dynamic programming.

Author: Erfaniaa

strings/is_substring_palindrome.py

@@ -0,0 +1,61 @@
+MAX_LEN = 1000
+
+text = ""
+dp = [[False for i in range(MAX_LEN)] for i in range(MAX_LEN)]
+
+def preprocess(s):
+    """
+        Preprocesses a string using dynamic programming.
+        Time complexity: O(n^2)
+    """
+    global text
+    global dp
+    text = s
+    dp = [[False for i in range(MAX_LEN)] for i in range(MAX_LEN)]
+    n = len(s)
+
+    for i in range(n):
+        dp[i][i] = True
+
+    for i in range(n - 1):
+        dp[i][i + 1] = (text[i] == text[i + 1]) 
+
+    for substr_len in range(2, n + 1):
+        for i in range(n - substr_len + 1):
+            j = i + substr_len - 1
+            dp[i][j] = (text[i] == text[j] and dp[i + 1][j - 1])
+
+
+def is_substring_palindrome(l, r):
+    """
+        Returns True if and only if the substring text[l:r] is a palindrome.
+        Time complexity: O(1)
+        Call preprocess function at least once, before calling this function.
+    """
+    n = len(text)
+    if l >= r or l < 0 or r > n:
+        return False
+    else:
+        return dp[l][r - 1]
+
+
+if __name__ == '__main__':
+    s = "'nursesrun' and 'racecar' are some palindromes."
+
+    preprocess(s)
+
+    # Answering some queries:
+    if is_substring_palindrome(1, 10):
+        print(s[1:10], "is a palindrome.")
+    else:
+        print(s[1:10], "is not a palindrome.")
+
+    if is_substring_palindrome(17, 24):
+        print(s[17:24], "is a palindrome.")
+    else:
+        print(s[17:24], "is not a palindrome.")
+
+    if is_substring_palindrome(35, 45):
+        print(s[35:45], "is a palindrome.")
+    else:
+        print(s[35:45], "is not a palindrome.")