main.cpp

// Source: https://leetcode.com/problems/best-time-to-buy-and-sell-stock
// Title: Best Time to Buy and Sell Stock
// Difficulty: Easy
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// You are given an array `prices` where `prices[i]` is the price of a given stock on the `i^th` day.
//
// You want to maximize your profit by choosing a **single day** to buy one stock and choosing a **different day in the future** to sell that stock.
//
// Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return `0`.
//
// **Example 1:**
//
// ```
// Input: prices = [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.
// Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.
// ```
//
// **Example 2:**
//
// ```
// Input: prices = [7,6,4,3,1]
// Output: 0
// Explanation: In this case, no transactions are done and the max profit = 0.
// ```
//
// **Constraints:**
//
// - `1 <= prices.length <= 10^5`
// - `0 <= prices[i] <= 10^4`
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <algorithm>
#include <climits>
#include <vector>

using namespace std;

// DP
//
// Store the previous lowest prices.
class Solution {
 public:
  int maxProfit(const vector<int>& prices) {
    int profit = 0;
    int minPrice = INT_MAX;
    for (int price : prices) {
      profit = max(profit, price - minPrice);
      minPrice = min(minPrice, price);
    }

    return profit;
  }
};