121_maxProfit.java

public class Solution {
    public int maxProfit(int[] prices) {
        if(prices == null || prices.length == 0) return 0;
        int min = prices[0];
        int profit = 0;
        for(int i = 1; i < prices.length; i++){
            if(prices[i] > min){
                profit = Math.max(profit, prices[i]-min);
            } else{
                min = prices[i];
            }
        }
        return profit;
    }
}