Description：
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 (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.
只允许买卖一次，求最大利润。
动态规划，从后向前找最大股价，减去当前股价，求利润，找最大利润。O(n).
public class Solution {
public int maxProfit(int[] prices) {
int len = prices.length;
if(len == 0) return 0;
int highestPrice = prices[len - 1];
int res = 0;
for(int i=len-1; i>=0; i--) {
highestPrice = max(highestPrice, prices[i]);
res = max(res, highestPrice-prices[i]);
}
return res;
}
int max(int a, int b) {
return a > b ? a : b;
}
}