121. Best Time to Buy and Sell Stock

问题描述
You are given an array prices where prices[i] is the price of a given stock on the ith day.

You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.

Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

采用dp的思想，先计算一遍盈利差，再计算一遍最大收益。
空间上还可以优化。

class Solution {
    public int maxProfit(int[] prices) {
        int best = 0;
        int[] difference = new int[prices.length];
        difference[0] = 0;
        for (int i = 1; i < prices.length; i++ ){
            difference[i] = prices[i] - prices[i - 1];
            if ( difference[i] + difference[i - 1] > difference[i] ){
                difference[i] = difference[i] + difference[i - 1];
            }
            if (difference[i] > best){
                best = difference[i];
            }
        }
        return best;
    }
}