adding algo

src/my_project/interviews/top_150_questions_round_22/ex_13_product_of_array_except_itself.py

@@ -0,0 +1,36 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        """
+        Calculate product of all elements except self without division.
+
+        Strategy: Two-pass with prefix and suffix products
+        - First pass: Build prefix products (product of all elements to the left)
+        - Second pass: Build suffix products (product of all elements to the right)
+        - Result[i] = prefix[i] * suffix[i]
+
+        Optimization: Use output array to store prefix, then multiply by suffix in-place
+
+        Time: O(n), Space: O(1) excluding output array
+        """
+
+        n = len(nums)
+        answer = [1] * n
+
+        # First pass: Calculate prefix products
+        # answer[i] contains product of all elements to the left of i
+        prefix = 1
+        for i in range(n):
+            answer[i] = prefix
+            prefix *= nums[i]
+
+        # Second pass: Calculate suffix products and multiply with prefix
+        # For each position, multiply existing prefix with product of all elements to the right
+        suffix = 1
+        for i in range(n - 1, -1, -1):
+            answer[i] *= suffix
+            suffix *= nums[i]
+
+        return answer

src/my_project/interviews/top_150_questions_round_22/ex_14_gas_station.py

@@ -0,0 +1,45 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class Solution:
+    def canCompleteCircuit(self, gas: List[int], cost: List[int]) -> int:
+        """
+        Find the starting gas station to complete the circuit.
+
+        Key insights:
+        1. If total gas < total cost, impossible to complete circuit
+        2. If we can't reach station j from station i, then we can't reach j 
+           from any station between i and j (they all have negative tank balance)
+        3. If a solution exists, it's guaranteed to be unique
+
+        Strategy: Greedy one-pass
+        - Track total gas surplus/deficit
+        - Track current tank from potential starting point
+        - If tank goes negative, reset start to next station
+
+        Time: O(n), Space: O(1)
+        """
+
+        n = len(gas)
+        total_gas = 0      # Total gas surplus/deficit for entire circuit
+        current_tank = 0   # Current gas in tank from start_station
+        start_station = 0  # Potential starting station
+
+        for i in range(n):
+            # Calculate net gas at this station (gain - cost to next)
+            net_gas = gas[i] - cost[i]
+
+            # Add to both total and current tank
+            total_gas += net_gas
+            current_tank += net_gas
+
+            # If current tank is negative, we can't reach next station from start_station
+            # All stations from start_station to i are invalid starting points
+            # Try starting from the next station (i + 1)
+            if current_tank < 0:
+                start_station = i + 1  # Reset starting point
+                current_tank = 0        # Reset tank
+
+        # If total gas is negative, impossible to complete circuit
+        # Otherwise, start_station is the answer
+        return start_station if total_gas >= 0 else -1

tests/test_150_questions_round_22/test_13_product_of_array_except_itself_round_22.py

@@ -0,0 +1,17 @@
+import unittest
+from src.my_project.interviews.top_150_questions_round_22\
+.ex_13_product_of_array_except_itself import Solution
+
+class ProductExcetItselfTestCase(unittest.TestCase):
+
+    def test_product_first_case(self):
+        solution = Solution()
+        output = solution.productExceptSelf(nums = [1,2,3,4])
+        target = [24,12,8,6]
+        self.assertEqual(output, target)
+
+    def test_product_second_case(self):
+        solution = Solution()
+        output = solution.productExceptSelf(nums = [-1,1,0,-3,3])
+        target = [0,0,9,0,0]
+        self.assertEqual(output, target)           

tests/test_150_questions_round_22/test_14_gas_station_round_22.py

@@ -0,0 +1,18 @@
+import unittest
+from src.my_project.interviews.top_150_questions_round_22\
+.ex_14_gas_station import Solution
+
+class GasStationTestCase(unittest.TestCase):
+
+    def test_gas_station_first_case(self):
+        solution = Solution()
+        output = solution.canCompleteCircuit(gas = [1,2,3,4,5], 
+                                             cost = [3,4,5,1,2])
+        target = 3 
+        self.assertEqual(output, target)
+
+    def test_gas_station_second_case(self):
+        solution = Solution()
+        output = solution.canCompleteCircuit(gas = [2,3,4], cost = [3,4,3])
+        target = -1 
+        self.assertEqual(output, target)