代码随性训练营第四十九天（Python）｜ 121. 买卖股票的最佳时机 、122.买卖股票的最佳时机II

121. 买卖股票的最佳时机
1、动态规划

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        # dp[i][0] 代表第 i 天持有股票获取的最大利益
        # dp[i][1] 代表第 i 天不持有股票获取的最大利益
        dp = [[0] * 2 for _ in range(len(prices))]
        dp[0][0] = - prices[0]
        dp[0][1] = 0
        for i in range(1, len(prices)):
            dp[i][0] = max(dp[i-1][0], -prices[i])
            dp[i][1] = max(dp[i-1][1], dp[i-1][0] + prices[i])
        return dp[-1][1]

2、贪心

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        res = 0
        low = prices[0]
        for i in range(1, len(prices)):
            low = min(low, prices[i])
            res = max(res, prices[i] - low)
        return res

3、动态规划优化

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        dp0, dp1 = - prices[0], 0
        for i in range(1, len(prices)):
            dp1 = max(dp1, dp0 + prices[i])
            dp0 = max(dp0, -prices[i])
        return dp1

122.买卖股票的最佳时机II
1、动态规划

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        # dp[i][0] 不持有股票，dp[i][1] 持有股票
        dp = [[0] * 2 for _ in range(len(prices))]
        dp[0][0] = 0
        dp[0][1] = - prices[0]
        for i in range(1, len(prices)):
            dp[i][0] = max(dp[i-1][0], dp[i-1][1] + prices[i])
            dp[i][1] = max(dp[i-1][1], dp[i-1][0] - prices[i])
        return dp[-1][0]