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121. Best Time to Buy and Sell Stock

題目​

完整題目

You are given an array prices where prices[i] is the price of a given stock on the ithi^{\text{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 \le prices.length \le 10^5 $
  • 0≤prices[i]≤1040 \le prices[i] \le 10^4

題目難易度​

Easy

解題想法​

找出股票賣買賣的時間點，並找出最大利潤。透過一次遍歷價格資料來持續更新購買的價格與最大利潤，最終找出結果。

DP​

  • Time Complexity: O(n)O(n)
  • Space Complexity: O(1)O(1)
/**
 * @param {number[]} prices
 * @return {number}
 */
var maxProfit = function (prices) {
  let buy = prices[0];
  let max = 0;

  for (let i = 0; i < prices.length; i++) {
    if (prices[i] < buy) buy = prices[i];
    max = Math.max(max, prices[i] - buy);
  }
  return max;
};