125. Valid Palindrome

Author: Shikha-code36

TwoPointers/125_Valid_Palindrome.py

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+'''
+125. Valid Palindrome
+Given a string s, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
+A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.
+
+Example 1:
+Input: s = "A man, a plan, a canal: Panama"
+Output: true
+Explanation: "amanaplanacanalpanama" is a palindrome.
+
+Example 2:
+Input: s = "race a car"
+Output: false
+Explanation: "raceacar" is not a palindrome.
+'''
+
+# Brute Force
+# Time Complexity: O(n)
+# Space Complexity: O(n)
+
+'''
+Below is the code for the brute force approach.
+It uses the built-in string methods to clean the input string and check if it is a palindrome.
+'''
+
+class Solution:
+    def isPalindrome(self, s):
+        clean = ''.join(c for c in s if c.isalnum()).lower()
+        return clean == clean[::-1]
+    
+# Two Pointers
+# Time Complexity: O(n)
+# Space Complexity: O(n)
+
+'''
+Below is the code for the two pointers approach.
+It uses two pointers to check if the cleaned string is a palindrome.
+The two pointers start at the beginning and end of the string and move towards the center, comparing characters along the way.
+If any characters don't match, it returns False. If all characters match, it returns True.
+'''
+
+class Solution:
+    def isPalindrome(self, s):
+        clean = ''.join(c for c in s if c.isalnum()).lower()
+        i=0 
+        j=len(clean)-1
+        
+        while i<=j:
+            if clean[i]!=clean[j]:
+                return False
+            else:
+                i+=1
+                j-=1
+        return True
+    
+# Two Pointers (Optimized)
+# Time Complexity: O(n)
+# Space Complexity: O(1)
+
+'''
+This approach uses the two-pointer technique to efficiently check if a string is a palindrome 
+while ignoring non-alphanumeric characters and treating uppercase and lowercase letters as the same
+'''
+
+class Solution:
+    def isPalindrome(self, s):
+        i=0
+        j=len(s)-1
+        
+        while i<j:
+            while i<j and not s[i].isalnum():
+                i+=1
+            while i<j and not s[j].isalnum():
+                j-=1
+            
+            if s[i].lower()!=s[j].lower():
+                return False
+            
+            i+=1
+            j-=1
+        return True
+    
+sol = Solution()
+s = "Was it a car or a cat I saw?"
+print(sol.isPalindrome(s))
+# Output: True