Added Stock Buy and Sell (Max One Transaction) solution in Python

Author: lavi445

data_structures/arrays/max_profit_one_transaction.py

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+# ---------------------------------------------------------------
+# Title: Stock Buy and Sell - Max One Transaction Allowed
+#
+# Problem:
+# Given an array prices[] representing the price of a stock on each day,
+# find the maximum profit achievable by performing at most one transaction.
+# (You must buy before you sell.)
+#
+# Example:
+# Input: prices = [7, 10, 1, 3, 6, 9, 2]
+# Output: 8   (Buy at 1, Sell at 9)
+#
+# Approach:
+# We traverse the list once, keeping track of:
+#   - min_price_so_far: The lowest price encountered so far.
+#   - max_profit: The maximum profit seen so far.
+#
+# For each price:
+#   - Compute potential profit = current_price - min_price_so_far.
+#   - Update max_profit if this profit is greater than the previous.
+#   - Update min_price_so_far if a smaller price is found.
+#
+# Time Complexity: O(n)
+# Space Complexity: O(1)
+# ---------------------------------------------------------------
+
+def max_profit(prices):
+    """
+    Calculate maximum profit from at most one buy-sell transaction.
+
+    Parameters:
+        prices (list[int]): List of stock prices per day.
+
+    Returns:
+        int: Maximum profit possible. Returns 0 if no profit is possible.
+    """
+
+    # Edge case: no transaction possible if list is too short
+    if not prices or len(prices) < 2:
+        return 0
+
+    # Initialize minimum price as the first day's price
+    min_price_so_far = prices[0]
+
+    # Initialize maximum profit
+    max_profit = 0
+
+    # Traverse price list starting from the second day
+    for price in prices[1:]:
+        # Update the minimum price seen so far
+        min_price_so_far = min(min_price_so_far, price)
+
+        # Calculate today's potential profit
+        profit_today = price - min_price_so_far
+
+        # Update max profit if today's profit is higher
+        max_profit = max(max_profit, profit_today)
+
+    return max_profit
+
+
+# ---------------------------------------------------------------
+# Example Usage (For quick testing)
+# ---------------------------------------------------------------
+if __name__ == "__main__":
+    prices = [7, 10, 1, 3, 6, 9, 2]
+    print("Stock Prices:", prices)
+    print("Maximum Profit:", max_profit(prices))  # Expected Output: 8