LeetCode - Best Time to Buy and Sell Stock - Day2 (1/2/2020)

LeetCode greedy

Best Time to Buy and Sell Stock

Question

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 (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Note that you cannot sell a stock before you buy one.

Example 1:

Input: [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 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
Explanation: In this case, no transaction is done, i.e. max profit = 0.

Solution

The solution said it is dp, but it doesn't seem dp question I guess.

The key concept is to keep track of min price and updating once we get max profit.

public int maxProfit(int[] prices) {
    int minPrice = Integer.MAX_VALUE;
    int maxProfit = 0;
    for (int i = 0; i < prices.length; i++) {
        if (prices[i] < minPrice) {
            minPrice = prices[i];
        } else if (prices[i] - minPrice > maxProfit) {
            maxProfit = prices[i] - minPrice;
        }
    }
    return maxProfit;
}

Time complexity

O(n) since we traverse the array.

Space complexity

O(1).