# Time: O(N)
# Space: O(1)
class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        if len(prices) < 1: return 0
        
        i, j = 0, 1
        max_profit = 0
        
        while i <= j < len(prices):
            max_profit = max(prices[j] - prices[i], max_profit)
            
            # Buy low, sell high — so we need to keep moving i (like
            # a buy pointer) if jth price is smaller than what ith is.
            if prices[j] < prices[i]:
                i = j
            else:
                j += 1
        
        return max_profit