Best Time to Buy and Sell Stock II

Problem Summary

(Write in your own words, not copied from LeetCode. This forces comprehension.)

• Find maximum profit, given list of prices
• Restriction is that you have to buy/sell each day

Key Observations

(Patterns, constraints, or hints in the problem statement.)

• Have to sell it or buy stock each day -> fixed window of size 1
• Calculate profit if sell price > buy price otherwise skip

Main Concepts Used

(Mark the CS concepts or algorithms used.)

• Sliding Window
• Array

Time & Space Complexity

• Time: $O(n)$ → Reason: Iterating through each item once
• Space: $O(1)$ → Reason: Not storing a copy of the array

Code

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        prev = None
        totalProfit = 0
        for price in prices:
            if prev != None and price > prev:
                profit = price - prev
                totalProfit += profit
            prev = price
        return totalProfit

Common Mistakes / Things I Got Stuck On

• if prev and ...: will fail because prev can be of value $0$
  • use explicit check prev != None