[SAM] 5th week solutions

3sum/samthekorean.py

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+# O(n^2) where there is nested loop.
+# O(1) where high and low are reused as constant values.
+
+
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        # Initialize a set to store unique triplets
+        triplets = set()
+
+        # Sort the list to make two-pointer approach possible
+        nums.sort()
+
+        # Iterate through the list, considering each element as a potential first element of a triplet
+        for i in range(len(nums) - 2):
+            low = i + 1
+            high = len(nums) - 1
+
+            # To avoid duplicate iteration
+            while low < high:
+                three_sum = nums[i] + nums[low] + nums[high]
+
+                if three_sum < 0:
+                    low += 1
+                elif three_sum > 0:
+                    high -= 1
+                else:
+                    triplets.add((nums[i], nums[low], nums[high]))
+                    low += 1
+                    high -= 1

encode-and-decode-strings/samthekorean.py

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+class Solution:
+    """
+    @param: strs: a list of strings
+    @return: encodes a list of strings to a single string.
+    """
+
+    # TC : O(n) where n is the combined length of the string in the list of strings.
+    # SC : O(S), where S is the sum of the lengths of all strings in strs
+    def encode(self, strs):
+        res = ""
+        for s in strs:
+            res += str(len(s)) + ":" + s
+        return res
+
+    """
+    @param: str: A string
+    @return: decodes a single string to a list of strings
+    """
+
+    # TC : O(n) where n is the length of encoded string
+    # SC : O(S), where S is the total length of the decoded strings.
+    def decode(self, str):
+        res, i = [], 0
+
+        while i < len(str):
+            j = i
+            while str[j] != ":":
+                j += 1
+            length = int(str[i:j])
+            res.append(str[j + 1 : j + length + 1])
+            i = j + 1 + length
+
+        return res
+
+
+def test_solution():
+    solution = Solution()
+
+    # Test case 1: Basic test case
+    input_strs = ["hello", "world"]
+    encoded = solution.encode(input_strs)
+    decoded = solution.decode(encoded)
+    print(f"Test case 1\nEncoded: {encoded}\nDecoded: {decoded}\n")
+
+    # Test case 2: Empty list
+    input_strs = []
+    encoded = solution.encode(input_strs)
+    decoded = solution.decode(encoded)
+    print(f"Test case 2\nEncoded: {encoded}\nDecoded: {decoded}\n")
+
+    # Test case 3: List with empty strings
+    input_strs = ["", "", ""]
+    encoded = solution.encode(input_strs)
+    decoded = solution.decode(encoded)
+    print(f"Test case 3\nEncoded: {encoded}\nDecoded: {decoded}\n")
+
+    # Test case 4: List with mixed empty and non-empty strings
+    input_strs = ["abc", "", "def"]
+    encoded = solution.encode(input_strs)
+    decoded = solution.decode(encoded)
+    print(f"Test case 4\nEncoded: {encoded}\nDecoded: {decoded}\n")
+
+    # Test case 5: List with special characters
+    input_strs = ["he:llo", "wo:rld"]
+    encoded = solution.encode(input_strs)
+    decoded = solution.decode(encoded)
+    print(f"Test case 5\nEncoded: {encoded}\nDecoded: {decoded}\n")
+
+
+test_solution()

product-of-array-except-self/samthekorean.py

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+# TC : O(n)
+# SC : O(1)
+
+
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        ans = [1] * (len(nums))
+
+        prefix = 1
+        for i in range(len(nums)):
+            ans[i] = prefix
+            prefix *= nums[i]
+        postfix = 1
+        for i in range(len(nums) - 1, -1, -1):
+            ans[i] *= postfix
+            postfix *= nums[i]
+
+        return ans

top-k-frequent-elements/samthekorean.py

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+# TC : O(n)
+# SC : O(n)
+from collections import defaultdict
+
+
+class Solution:
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+        # Initialize a list of empty lists to store numbers based on their frequencies
+        freq = [[] for i in range(len(nums) + 1)]
+
+        # Dictionary to count the frequency of each number
+        counter = defaultdict(int)
+
+        # Count the frequency of each number in nums
+        for num in nums:
+            counter[num] += 1
+
+        # Group numbers by their frequency
+        for n, c in counter.items():
+            freq[c].append(n)
+
+        # Result list to store the top k frequent elements
+        res = []
+
+        # Iterate over the frequency list in reverse order to get the most frequent elements first
+        for i in range(len(freq) - 1, 0, -1):
+            for n in freq[i]:
+                res.append(n)
+                # Once we have k elements in the result, return it
+                if len(res) == k:
+                    return res