diff --git a/clone-graph/imgolden77.py b/clone-graph/imgolden77.py
new file mode 100644
index 0000000000..9c506de4cb
--- /dev/null
+++ b/clone-graph/imgolden77.py
@@ -0,0 +1,33 @@
+"""
+# Definition for a Node.
+class Node:
+    def __init__(self, val = 0, neighbors = None):
+        self.val = val
+        self.neighbors = neighbors if neighbors is not None else []
+"""
+
+from typing import Optional
+class Solution:
+    def cloneGraph(self, node: Optional['Node']) -> Optional['Node']:
+        if not node:
+            return None
+        old_to_new = {}
+
+        def dfs(curr_node):
+            if curr_node in old_to_new:
+                return old_to_new[curr_node]
+            
+            copy = Node(curr_node.val)
+            old_to_new[curr_node] = copy
+
+            for neighbors in curr_node.neighbors:
+                copy.neighbors.append(dfs(neighbors))
+
+            return copy
+
+        return dfs(node)
+    
+    # O(N+E) time complexity where N is the number of nodes and E is the number of edges in the graph.
+    # O(N) space complexity for the hashmap and the recursion stack in the worst case.
+    
+    
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diff --git a/longest-repeating-character-replacement/imgolden77.py b/longest-repeating-character-replacement/imgolden77.py
new file mode 100644
index 0000000000..65b7a615d9
--- /dev/null
+++ b/longest-repeating-character-replacement/imgolden77.py
@@ -0,0 +1,28 @@
+class Solution:
+    def characterReplacement(self, s: str, k: int) -> int:
+        start = 0
+        max_len = 0
+        seen = {}
+
+        for i, char in enumerate(s):
+
+            seen[char] = seen.get(char, 0) + 1
+            # what I wrote initially:     
+            # if char not in seen:
+            #     seen[char] = 1
+            # else:
+            #     seen[char] += 1
+            max_val = max(list(seen.values()))
+            window_len = i - start + 1
+            if window_len - max_val  <= k:
+                max_len = max(max_len, window_len)
+            else:
+                seen[s[start]] -= 1
+                start += 1
+
+        return max_len
+    
+    # O(N) time complexity and O(1) space complexity.
+
+
+    
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diff --git a/longest-substring-without-repeating-characters/imgolden77.py b/longest-substring-without-repeating-characters/imgolden77.py
new file mode 100644
index 0000000000..fd145d2a11
--- /dev/null
+++ b/longest-substring-without-repeating-characters/imgolden77.py
@@ -0,0 +1,32 @@
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        h = []
+        max_s = 0
+        for i in s:
+            if i not in h:
+                h.append(i)
+            else:
+                h=h[h.index(i)+1:]
+                h.append(i)
+                #I reset h=[] but it had problemswith the cases like "dvdf"
+                #So I modified it to h=h[h.index(i)+1:]
+            max_s = max(max_s, len(h))
+        
+        return max_s
+
+#O(N^2) time complexity due to "if i not in h" and O(min(m,n)) space complexity where m is the size of the charset and n is the size of the string s.
+
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        seen = {}
+        start = 0
+        max_len = 0
+        for i, char in enumerate(s):
+            if char in seen and seen[char]>=start:
+                start= seen[char]+1    
+            seen[char]=i
+            max_len = max(max_len, i - start +1)
+            
+        return max_len
+#O(N) time complexity and O(min(m,n)) space complexity where m is the size of the charset and n is the size of the string s.
+#Dictionary search and insert operations are O(1) on average. so it is O(N) overall.
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diff --git a/number-of-islands/imgolden77.py b/number-of-islands/imgolden77.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/palindromic-substrings/imgolden77.py b/palindromic-substrings/imgolden77.py
new file mode 100644
index 0000000000..c169fa7ace
--- /dev/null
+++ b/palindromic-substrings/imgolden77.py
@@ -0,0 +1,25 @@
+class Solution:
+    def countSubstrings(self, s: str) -> int:
+        total_count = 0
+
+        def expand(left, right):
+            local_count = 0
+            while left>=0 and right< len(s) and s[left] == s[right]:
+            # while s[left] == s[right] and left>=0 and right< len(s) :
+            # The above line would cause index error if left or right go out of bounds
+                local_count += 1
+                left -= 1
+                right += 1
+            return local_count
+
+        for i in range(len(s)):
+            total_count += expand(i, i)
+            total_count += expand(i, i+1)
+        
+        return total_count
+    
+#Complexity Analysis
+#Time complexity: O(n^2) where n is the length of the input string s.
+#Space complexity: O(1) as we are using only a constant amount of extra space
+# Manacher's Algorithm can achieve O(n) time complexity for this problem.
+
diff --git a/reverse-bits/imgolden77.py b/reverse-bits/imgolden77.py
new file mode 100644
index 0000000000..e897d95634
--- /dev/null
+++ b/reverse-bits/imgolden77.py
@@ -0,0 +1,16 @@
+class Solution:
+    def reverseBits(self, n: int) -> int:
+        if n == 0:
+            return 0
+        res = 0
+        for i in range(32):
+            bit = n & 1
+            res = res + bit
+            res = res << 1
+            n = n >> 1
+        res = res >> 1
+            
+        return res
+    
+# Complexity O(1) - Time: 32 iterations, Space: O(1) no extra space used
+
diff --git a/reverse-linked-list/imgolden77.py b/reverse-linked-list/imgolden77.py
new file mode 100644
index 0000000000..4a32522286
--- /dev/null
+++ b/reverse-linked-list/imgolden77.py
@@ -0,0 +1,20 @@
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        curr = head
+        prev = None
+        
+        while curr is not None: # once was "while curr.next is not None:" but that showed AttributeError: 'NoneType' object has no attribute 'next'
+            if prev:
+                post = curr.next
+                curr.next = prev
+                prev = curr
+                curr = post
+            else:
+                post = curr.next
+                prev = curr
+                curr.next = None
+                curr = post
+
+        return prev
+    
+# First time, return Curr, but the result was [] so I changed to "return prev" as the new head of the reversed Linked list.
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diff --git a/valid-parentheses/imgolden77.py b/valid-parentheses/imgolden77.py
new file mode 100644
index 0000000000..76e4aa4045
--- /dev/null
+++ b/valid-parentheses/imgolden77.py
@@ -0,0 +1,57 @@
+# First try O(n) time complexity
+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = []
+        former= ['(', '{', '[']
+        
+        for i in s:
+            if i in former:
+                stack.append(i)
+            elif i == ')':
+                if stack== []:
+                    return False
+                elif stack[-1]== '(':
+                    stack.pop()
+                else:
+                    return False
+            elif i == '}':
+                if stack== []:
+                    return False
+                elif stack[-1]== '{':
+                    stack.pop()
+                else:
+                    return False
+            elif i == ']':
+                if stack == []:
+                    return False
+                elif stack[-1]== '[':
+                    stack.pop()
+                else:
+                    return False
+            
+        
+        if stack == []:
+            return True
+        else:
+            return False
+        
+# Second try: O(n) time complexity
+# refer other people's code 
+# 'not stack' is more pythonic way to check empty stack
+# par_map dictionary to map closing to opening parentheses
+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = []
+        par_map = {')':'(', '}':'{', ']':'['}
+        
+        for i in s:
+            if i in par_map.values():
+                stack.append(i)
+            elif i in par_map:
+                if not stack or stack[-1] !=par_map[i]:
+                    return False
+                stack.pop()
+            else:
+                return False
+        
+        return not stack
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