leetcode121、122、123

今天发现leetcode比别的都好。。。因为面试面到121题，我愚笨的脑袋用了两个for循环，只用一个for循环我是想不出的。
121:

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        size_t size = prices.size();
        if(size <= 1)   return 0;
        int min = INT_MAX, max = INT_MIN;
        for(int i=0; i<size; i++) {
            if(min > prices[i]) min = prices[i];
            if(max < prices[i] - min)   max = prices[i] - min;
        }
        return max;
    }
};

122:

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

123:

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int size = prices.size();
        if(size <= 1)   return 0;

        int *ascandMax = new int[size]();
        int minPrice = prices[0];
        int maxProf = 0;
        for(int i=0; i<size; i++) {
            maxProf = max(maxProf, prices[i]-minPrice);
            minPrice = min(minPrice, prices[i]);
            ascandMax[i] = maxProf;
        }

        int *descandMax = new int[size]();
        int maxPrice = prices[size-1];
        maxProf = 0;
        for(int i=size-2; i>=0; i--) {
            maxProf = max(maxProf, maxPrice-prices[i]);
            maxPrice = max(maxPrice, prices[i]);
            descandMax[i] = maxProf;
        }

        maxProf = 0;
        for(int i=0; i<size; i++) {
            maxProf = max(maxProf, ascandMax[i] + descandMax[i]);
        }

        delete[] ascandMax;
        delete[] descandMax;
        return maxProf;
    }
};