Best Time to Buy and Sell Stock I

Say you have an array for which the i_th 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.

Solution:

int maxProfit(vector<int>& prices) {
        vector<int> dp(prices);
        int ret = 0;
        for(int i=1;i<prices.size();i++){
            dp[i] = dp[i-1]; 
            if(prices[i]-dp[i-1]>ret){
                ret = prices[i]-dp[i-1]; 
            }
            if(prices[i]-dp[i-1]<0){
                dp[i] = prices[i];  
            }
        }
        return ret;
}

Best Time to Buy and Sell Stock II:

Say you have an array for which the i_th element is the price of a given stock on day _i.(joke question...)

Design an algorithm to find the maximum profit. You may complete as many transactions as you like (i.e., buy one and sell one share of the stock multiple times).

int maxProfit(vector<int>& prices) {
        int ret=0;
        if(prices.size()==0) return 0;
        for(int i=0;i<prices.size()-1;i++){
            if(prices[i+1]>prices[i]){
                ret+=prices[i+1]-prices[i];
            }
        }
        return ret;
    }