http://oj.leetcode.com/problems/best-time-to-buy-and-sell-stock/
class Solution {
public:
int maxProfit(vector<int> &prices) {
// Note: The Solution object is instantiated only once and is reused by each test case.
if(prices.size()<=1) return 0;
int res=0;
int least=prices[0];
for(int i=1;i<prices.size();i++){
res=max(prices[i]-least,res);
least=min(prices[i],least);
}
return res;
}
};