[LeetCode] 121. Best Time to Buy and Sell Stock

Say you have an array for which the ith element is the price of a given stock on day i.

If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Example 1:

Input: [7, 1, 5, 3, 6, 4]
Output: 5

max. difference = 6-1 = 5 (not 7-1 = 6, as selling price needs to be larger than buying price)

Example 2:

Input: [7, 6, 4, 3, 1]
Output: 0

In this case, no transaction is done, i.e. max profit = 0.

Thought process:
Iterate through the array. Keep track of lowest price, and max profit. This works because we can only sell after buying the stock.

• If current price is less than the lowest price, update lowest price.
• Else, calculate profit with lowest price and update max profit.

Solution:

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class Solution { public int maxProfit(int[] prices) { int max = 0; int lowest = Integer.MAX_VALUE; for (int price : prices) { if (price < lowest) { lowest = price; } else { max = Math.max(max, price - lowest); } } return max; } }

Time complexity: O(n).