---
title: 'LeetCode 121. Best Time to Buy and Sell Stock'
disqus: hackmd
---

# LeetCode 121. Best Time to Buy and Sell Stock

## Description

You are given an array prices where prices[i] is the price of a given stock on the ith 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

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.

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^

## Answer

此題用DP去解，先取第一個為buy，然後一個一個往下看，若有利潤就暫存並更新最大利潤，若無表示有更低價，更新buy。

// 2021_11_08
int maxProfit(int* prices, int pricesSize) {
    if(pricesSize < 2){return 0;}
    int buy = *prices, max = 0, profit = 0, i = 0;
    for(i = 1; i<pricesSize; i++){
        profit = prices[i] - buy;
        buy = profit > 0 ? buy : prices[i];
        max = max > profit ? max : profit;
    }
    return max;
}

## Link
https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
###### tags: `Leetcode`