Best Time to Buy and Sell Stock

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121.Best Time to Buy and Sell Stock

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 1:

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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:

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Input: prices = [7,6,4,3,1]
Output: 0
Explanation: In this case, no transactions are done and the max profit = 0.

时间复杂度：O(n)

空间复杂度：O(1)

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class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int max = prices[0];
        int min = prices[0];
        int ans = 0;
        for (int price : prices) {
            if (price < min) {
                min = price;
                max = price;
            }
            if (price > max)max = price;
            if (ans < (max - min))ans = max - min;
        }
        return ans;
    }
};