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/**
 * hold: the money status when holding the stock.
 * sold: the money status after selling the stock.
 */
class Solution {
    public int maxProfit(int[] prices, int fee) {
        int hold = -prices[0], sold = 0;
        for (int price:prices) {
            int pre_hold = hold; // memorize hold in previous day to calculate the sold today. 
            hold = Math.max(hold, sold - price);
            sold = Math.max(sold, pre_hold + price - fee);
        }
        return sold;
    }
}