Added Two_Sum and Longest_Palindromic_Substring

Python/1_TwoSum.py

@@ -0,0 +1,56 @@
+#Difficulty = Easy
+#Submission Speed = 64.69% 
+'''
+Solution-1:
+Example: nums = [3,2,1,4]       target = 6
+Step1) Create a dictionary and populate it with elements of nums as the key and their corresponding index as values.
+
+    d = {
+            3:0,
+            2:1
+            1:2
+            4:3
+        }
+Step2) Traverse the array and check for every element, if complement (target-element) exists in dictionary. Also check,
+the index of (target-element) is not same as that of element (using dictionary as we have saved index as values.)
+Traversing the array:
+#   i=0         3,2,1,4
+                ^
+    checking if (6-3) exists in dictionary?
+    Yes it exists
+    but d[6-3]==i (We are adding the same element to get the target which is invalid according to the question)
+
+Step3) If we found a valid pair, then we return [i,d[complement]]
+'''
+'''
+Time Complexity =  O(N)
+Space Complexity = O(N)
+'''
+
+#Two-pass
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        d = {v:i for i,v in enumerate(nums)}
+        for i in range(len(nums)):
+            complement = target - nums[i]
+            if complement in d and d[complement]!=i:
+                return [i,d[complement]]
+
+'''
+Solution-2: Single Pass:
+Instead of doing double passes (one pass while populating the dictionary and other while checking for complement),
+We do this in Single Pass. that is, checking for complement while populating the dictionary.
+Before inserting the element in dictionary, we check whether the complement already exists, if exists, we return both indexes
+(index of current element and index of its complement) and if not exists, we insert the element in dictionary.
+This way we wont have to check if the index of element and its complement is same or not.
+'''
+
+#Single Pass
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        d = {}
+        for i in range(len(nums)):
+            complement = target - nums[i]
+            if complement in d:
+                return [d[complement],i]
+            d[nums[i]]=i

Python/5_LongestPalindromicSubstring.py

@@ -0,0 +1,46 @@
+#Difficulty = Medium
+#Submission Speed = 48.40%
+'''
+Before Jumping onto Solution, know that:
+=>Palindromes are of two types: Odd Palindromes (length is odd) and Even Palindrome (length is even)
+=>We traverse the array, and for every element, we suppose that current element is the middle element and check for the largest palindromic
+  substring that can be made which has middle element as the current element.
+Note that, In case of even palindrome, there are two middle elements.
+=> We check for both Odd and even Palindromes
+=> How to check for largest palindromic substring with given middle element?
+    we maintain two pointers left and right.
+    Initialize them with current index (middle element index) and In every loop we check s[left]==s[right] (and check for 
+    boundary errors too, left and right dont go out of the size of array or 0). In the body of the loop we decrease the left
+    and increase the right pointer.
+'''
+'''
+Time Complexity =  O(N^2)
+Space Complexity = O(N)
+'''
+
+
+class Solution:
+    def longestPalindrome(self, s: str) -> str:
+        longest = ""
+        size = len(s)
+        for i in range(size):
+            #Odd Palindrome
+            left,right = i,i
+            ls = ""
+            while 0<=left<size and 0<=right<size and s[left]==s[right]:
+                if left==right:
+                    ls = s[left]
+                else:
+                    ls = s[left] + ls + s[right]
+                left-=1
+                right+=1
+            #Even Palindrome
+            left,right = i,i+1
+            rs = ""
+            while 0<=left<size and 0<=right<size and s[left]==s[right]:
+                rs = s[left] + rs + s[right]
+                left-=1
+                right+=1
+            temp = ls if len(ls)>len(rs) else rs
+            longest = temp if len(temp)>len(longest) else longest
+        return longest

README.md

@@ -127,6 +127,19 @@
 | #   | Title                                                         | Solution                          | Time   | Space  | Difficulty | Tag | Note          |
 | --- | ------------------------------------------------------------- | --------------------------------- | ------ | ------ | ---------- | --- | ------------- |
 | 242 | [Valid Anagram](https://leetcode.com/problems/valid-anagram/) | [Java](./Java/valid-anagram.java) | _O(n)_ | _O(1)_ | Easy       |     | Unicode chars |
+|1| [Two Sum](https://leetcode.com/problems/two-sum/)| [Python](./Python/1_TwoSum.py)|_O(N)_|_O(N)_|Easy|||
+
+<br/>
+<div align="right">
+    <b><a href="#algorithms">⬆️ Back to Top</a></b>
+</div>
+<br/>
+
+## Two Pointer
+
+| #    | Title                                                                                               | Solution                                                | Time     | Space     | Difficulty | Tag   | Note           |
+| ---- | --------------------------------------------------------------------------------------------------- | ------------------------------------------------------- | -------- | --------- | ---------- | ----- | -------------- |
+|5|[Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/)|[Python](./Python/5_LongestPalindromicSubstring.py)|_O(N^2)_|_O(N)_|Medium||Expand the Wings|
 
 <br/>
 <div align="right">