題目：
Say you have an array for which the ith element is the price of a given stock on day i.
If you were only permitted to complete at most one transaction (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit.
Note that you cannot sell a stock before you buy one.
Example 1:
Input: [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5.
Not 7-1 = 6, as selling price needs to be larger than buying price.
Example 2:
Input: [7,6,4,3,1]
Output: 0
Explanation: In this case, no transaction is done, i.e. max profit = 0.
股票買賣，只能操作一次，所以要在最低價買進，最高價賣出，且買進在賣出前。
用一個變量記錄當前價格前的最低價，算出今天如果賣出的話可以賺到的最大利潤。遍歷整個數組，更新這個值，同時更新最大利潤。
AC:
public int maxProfit(int[] prices) {
if (prices == null || prices.length == 0) {
return 0;
}
int max = 0;
int min = prices[0];
int size = prices.length;
for (int i = 1; i < size; i++) {
int profit = prices[i] - min;
if (profit > max) {
max = profit;
}
if (prices[i] < min) {
min = prices[i];
}
}
return max;
}