```
class Solution:
# @param {integer[]} prices
# @return {integer}
def maxProfit(self, prices):
if prices is None:
return 0
p_len = len(prices)
if p_len <=1:
return 0
p_min = prices[0]
pr_max = 0
for i in prices:
if i >= p_min:
pr = i - p_min
if pr > pr_max:
pr_max = pr
else:
p_min = i
return pr_max
```