O(n) time O(1) space one pass solution

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