面试经典150题——Day7

一、题目

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:

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 <= 105
0 <= prices[i] <= 104

题目来源：leetcode

二、题解

使用动态规划的思想，避免双重for循环，在O(n)时间内解决问题

class Solution {
public:
    int max(int a,int b){
        if(a > b) return a;
        else return b;
    }
    int maxProfit(vector<int>& prices) {
        const int N = 100010;
        int n = prices.size();
        int dp[N];
        dp[0] = 0;
        int minVal = prices[0];
        for(int i = 1;i < n;i++){
            if(prices[i] < minVal) minVal = prices[i];
            dp[i] = max(prices[i] - minVal,dp[i-1]);
        }
        return dp[n-1];
    }
};