121. Best Time to Buy and Sell Stock

顺着数组走，保存：

1.到目前为止最大profit

2.到目前为止最小price

更新两个数据，结尾返回maxProfit

   public int maxProfit(int[] prices) {
        if(prices.length < 1) {
            return 0;
        }
        int minPrice = prices[0];
        int maxProfit = 0;
        for(int i = 1; i < prices.length; i++) {
            minPrice = (prices[i] < minPrice)? prices[i]: minPrice;
            maxProfit = (maxProfit < prices[i] - minPrice)? prices[i] - minPrice: maxProfit;
        }
        return maxProfit;
    }