add : Best time to buy and sell stock program under GREEDY methods

greedy_methods/best_time_to_buy_and_sell_stock.py

@@ -0,0 +1,47 @@
+"""
+Given a list of stock prices,
+This program calculates the maximum profit that can be made from a single buy and sell.
+We can only complete one transaction (buy one and sell one share of the stock).
+We must buy before we can sell.
+
+Example : prices = [7, 1, 5, 3, 6, 4]
+max_profit will return 5 - which is by buying at price 1 and selling at price 6.
+
+This problem can be solved using the concept of "GREEDY ALGORITHM".
+
+We iterate over the price array once, keeping track of the lowest price point
+(buy) and the maximum profit we can get at each point.
+The greedy choice at each point is to either buy at the current price if
+it's less than our current buying price, or sell at the current price if the profit
+is more than our current maximum profit.
+"""
+
+
+def max_profit(prices: list[int]) -> int:
+    """
+    >>> max_profit([7, 1, 5, 3, 6, 4])
+    5
+    >>> max_profit([7, 6, 4, 3, 1])
+    0
+    """
+    if not prices:
+        return 0
+
+    min_price = prices[0]
+    max_profit: int = 0
+
+    for price in prices:
+        if price < min_price:
+            min_price = price
+        elif price - min_price > max_profit:
+            max_profit = price - min_price
+
+    return max_profit
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
+    print(max_profit([7, 1, 5, 3, 6, 4]))