121. 买卖股票的最佳时机

Author: chenpengcode

dp/121_maxProfit.py

@@ -2,6 +2,66 @@
 
 
 class Solution:
+    def maxProfit_1(self, prices: List[int]) -> int:
+        if not prices:
+            return 0
+        max_profit = 0
+        for i in range(len(prices)):
+            for j in range(i + 1, len(prices)):
+                max_profit = max(max_profit, prices[j] - prices[i])
+        return max_profit
+
+    def maxProfit_dp(self, prices: List[int]) -> int:
+        if not prices:
+            return 0
+        n = len(prices)
+        # 创建两个数组，一个记录每次卖出的最大收益，一个记录每次买入最大收益
+        buy = [0] * n
+        sell = [0] * n
+        buy[0] = -prices[0]
+        for i in range(1, n):
+            # 第i天卖出收益 = max(第i - 1天卖出收益，第i - 1天买入收益 + 当天股价)
+            sell[i] = max(sell[i - 1], buy[i - 1] + prices[i])
+            # 第i天买入收益 = max(第i - 1天买入收益，-当天股价)
+            buy[i] = max(buy[i - 1], -prices[i])
+        return sell[-1]
+
+    def maxProfit_dp_1(self, prices: List[int]) -> int:
+        if not prices:
+            return 0
+        n = len(prices)
+        dp = [[0 for _ in range(2)] for _ in range(n)]
+        dp[0][0] = 0
+        dp[0][1] = -prices[0]
+        for i in range(1, n):
+            dp[i][0] = max(dp[i - 1][0], dp[i - 1][1] + prices[i])
+            dp[i][1] = max(dp[i - 1][1], -prices[i])
+        return dp[-1][0]
+
+    def maxProfit_dp_2(self, prices: List[int]) -> int:
+        if not prices:
+            return 0
+
+        n = len(prices)
+        sell = 0
+        buy = -prices[0]
+        for i in range(1, n):
+            sell = max(sell, buy + prices[i])
+            buy = max(buy, -prices[i])
+        return sell
+
+    def maxProfit_greed(self, prices: List[int]) -> int:
+        if not prices:
+            return 0
+        n = len(prices)
+        min_price = prices[0]
+        max_profit = 0
+        for i in range(1, n):
+            max_profit = max(max_profit, prices[i] - min_price)
+            min_price = min(min_price, prices[i])
+        return max_profit
+
+    # =============================================
     def maxProfit(self, prices: List[int]) -> int:
         if not prices:
             return 0
@@ -42,4 +102,4 @@ def maxProfit_better(self, prices: List[int]) -> int:
 if __name__ == '__main__':
     prices = [7, 1, 5, 3, 6, 4]
     solution = Solution()
-    print(solution.maxProfit_better(prices))
+    print(solution.maxProfit_greed(prices))