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

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Problem

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:

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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:

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Input: [7,6,4,3,1]
Output: 0
Explanation: In this case, no transaction is done, i.e. max profit = 0.

Explanation

  1. While we iterating the array, we need to keep track of the minimum element, and in each iteration, we subtract the minimum to get the current profit.

  2. Also, each iteration we need to compare the maximum profit with the current profit.

Solution

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class Solution {
    public int maxProfit(int[] prices) {
        int minPrice = Integer.MAX_VALUE;
        int profit = 0;
        for (int i = 0; i < prices.length; i++) {
            minPrice = Math.min(minPrice, prices[i]);
            profit = Math.max(profit, prices[i] - minPrice);
        }
        return profit;
    }
}