动态规划1

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        # 数组长度
        n = len(prices)
        if n < 2:
            return 0
        # 动态规划变量
        # 第一次买的价格
        first_price = prices[0]
        # 第一次卖的收益
        first_cell = 0
        # 第二次买的价格
        second_price = prices[0]
        # 第二次卖
        second_cell = 0
        # 开始动态规划迭代
        for i in range(1, n):
            first_price = min(first_price, prices[i])
            first_cell = max(first_cell, prices[i]-first_price)
            second_price = min(second_price, prices[i]-first_cell)
            second_cell = max(second_cell, prices[i]-second_price)
        return second_cell

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动态规划2

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        # 数组长度
        n = len(prices)
        if n < 2:
            return 0
        # 动态规划变量
        # 第一次买的价格
        first_price = prices[0]
        # 第一次卖的收益
        first_cell = 0
        # 第二次买的价格
        second_price = prices[0]
        # 第二次卖
        second_cell = 0
        # 开始动态规划迭代
        for i in range(1, n):
            first_price = min(first_price, prices[i])
            first_cell = max(first_cell, prices[i]-first_price)
            second_price = min(second_price, prices[i]-first_cell)
            second_cell = max(second_cell, prices[i]-second_price)
        return second_cell

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动态规划3

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        # 数组长度
        n = len(prices)
        if n < 2:
            return 0
        # 动态规划变量
        # 第一次买
        pre_first_buy = -prices[0]
        # 第一次卖
        pre_first_cell = 0
        # 第二次买
        pre_second_buy = -prices[0]
        # 第二次卖
        pre_second_cell = 0
        # 开始动态规划迭代
        for i in range(1, n):
            first_buy = max(pre_first_buy, -prices[i])
            first_cell = max(pre_first_cell, pre_first_buy+prices[i])
            second_buy = max(pre_second_buy, pre_first_cell-prices[i])
            second_cell = max(pre_second_cell, pre_second_buy+prices[i])
            pre_first_buy = first_buy
            pre_first_cell = first_cell
            pre_second_buy = second_buy
            pre_second_cell = second_cell
        return second_cell