309. Best Time to Buy and Sell Stock with Cooldown
题目大意：给定一个数组，第i个元素表示第i天的股票价格。每天只能最多一次买或者卖(必须有股票才能卖)。卖了以后第二天不能买(cooldown)。求最大的利润。
解题思路：
动态规划
class Solution(object):
def maxProfit(self, prices):
sold = 0
rest = 0
hold = -float('inf')
for price in prices:
prev_sold = sold
sold = hold + price
hold = max(hold, rest - price)
rest = max(rest, prev_sold)
return max(rest, sold)