121. Best Time to Buy and Sell Stock 最佳买卖股票时间

Say you have an array for which the ith element is the price of a given stock on dayi.

If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Example 1:

Input: [7, 1, 5, 3, 6, 4]
Output: 5

max. difference = 6-1 = 5 (not 7-1 = 6, as selling price needs to be larger than buying price)

Example 2:

Input: [7, 6, 4, 3, 1]
Output: 0

In this case, no transaction is done, i.e. max profit = 0.

class Solution {
public int maxProfit(int[] prices) {
if(prices==null||prices.length<=1){
return 0;
}
int buy = prices[0];
int profit = 0;
for(int i = 0; i < prices.length; i++){
if(prices[i] < buy){
buy = prices[i];
}
if(prices[i] - buy > profit){
profit = prices[i] - buy;
}
}
return profit;
}
}

解题思路：

把买入的价格和利润初始化

循环遍历，若当前价格小于买入价格，则重置买入价格为当前价格；若当前价格大于买入价格，且考虑当前价格是否可作为卖出价格，将当前价格和买入价格的差值与利润作比较，选较大值赋值给利润