121.BestTimeToBuyAndSellStock.cpp

/*
 * @lc app=leetcode id=121 lang=cpp
 *
 * [121] Best Time to Buy and Sell Stock
 *
 * https://leetcode.com/problems/best-time-to-buy-and-sell-stock/description/
 *
 * algorithms
 * Easy (50.37%)
 * Likes:    5431
 * Dislikes: 237
 * Total Accepted:    907.3K
 * Total Submissions: 1.8M
 * Testcase Example:  '[7,1,5,3,6,4]'
 *
 * 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.
 * 
 * 
 */

// @lc code=start
#include <vector>
#include <algorithm>

class Solution {
public:
    int maxProfit(std::vector<int>& prices) {
        if (prices.empty()) return 0;
        
        int n_days = prices.size();
        int max_profit = 0;
        
        int min_price_before = prices[0];
        
        for (int i = 1; i < n_days; i++) {
            max_profit = std::max(max_profit, prices[i] - min_price_before);
            min_price_before = std::min(min_price_before, prices[i]);
        }
        
        return max_profit;
    }
};
// @lc code=end