121 Best Time to Buy and Sell Stock

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.

Example 1:

Input: [7, 1, 5, 3, 6, 4]
Output: 5

max. difference = 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

In this case, no transaction is done, i.e. max profit = 0.

Solution:

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int n = prices.size();
        if ( n <= 1 ) return 0;
        int minP = prices[0], maxP = 0;
        for ( int i = 1; i <= n-1; i++ ) {
            minP = min(minP, prices[i]);
            maxP = max(maxP, prices[i] - minP);
        }
        return maxP;
    }
};