和大神们学习每天一题(leetcode)-Best Time to Buy and Sell Stock

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

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.

本题主要在于寻找低谷和高峰

功能测试用例：{ 1,4,11,1,9}

特殊输入测试用例：NULL, {1},{1,2,3,4,5},{5,4,3,2,1}

class Solution
{
public:
int maxProfit(vector<int> &prices)
{
if (prices.size() < 2)//如果价格个数小于2则返回空
return NULL;
int  nMaxPro = 0, nLower = prices[0], nUpper = prices[0];//初始化最高和最低值为第一天的价格
for (int nTemp = 0; nTemp < prices.size() - 1; nTemp++)
{
if (prices[nTemp] <= prices[nTemp + 1])//判断价格是否上升
{
if (prices[nTemp + 1] > nUpper)//判断当前价格是否高于最高值
{
nUpper = prices[nTemp + 1];//将最高值赋为当前值
if (nUpper - nLower > nMaxPro)//算出当前差价最高值
nMaxPro = nUpper - nLower;
}
}
else//价格下降或不变
{
if (prices[nTemp + 1] < nLower)//判断当前价格是否低于最低值
{
nLower = prices[nTemp + 1];
nUpper = nLower;
}
}
}
return nMaxPro;
}
};