```
// f(i) is the profit of the ith day
// f(i) = f(i-1) + (a[i]-a[i-1]), f(n-1)>0
// f(i) = a[i]-a[i-1], f(n-1)<=0
// i = 1...n
public class Solution {
public int maxProfit(int[] prices) {
if(prices ==null || prices.length <2) return 0;
int profit = 0;
int max =0;
int len = prices.length;
for(int i=1;i<len;i++){
if(profit <= 0){
profit = prices[i]-prices[i-1];
}else{
profit += (prices[i]-prices[i-1]);
}
if(profit > max) max = profit;
}
return max;
}
}
```