LeetCode 121

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        if(prices == nullptr || prices.size() < 1)
            return 0;
        int max_profit = 0;
        for( int i = 0; i < prices.size() ; ++i){
            for(int j = i+1;j<prices.size(); ++j)
                if(prices[j] - prices[i] > max_profit)
                    max_profit = prices[j] - prices[i];
        }
        return max_profit;
    }
};


class Solution {
public:
    int maxProfit(vector<int>& prices) {
        if(prices == nullptr || prices.size() < 1)
            return 0;
        int max_profit = 0;
        int min = prices[0];
        for( int i = 1; i < prices.size() ; ++i){
            if(min > prices[i]) min = prices[i];
            if(max_profit < prices[i] - min) max_profit = prices[i]-min;
        }
        return max_profit;
    }
};

维护min来加快速度，一遍循环。