628. Maximum Product of Three Numbers

问题：求一个数列里三个数相乘的最大值

Input: [1,2,3]
Output: 6

Input: [1,2,3,4]
Output: 24

Note: 
1.The length of the given array will be in range [3,104] and all elements are in the range [-1000, 1000].
2.Multiplication of any three numbers in the input won't exceed the range of 32-bit signed integer.

方针：

最大乘积 =

最大的3个数相乘

or

最小的两个数（两个负数）*最大的数

代码参考：

class Solution {
public:
    int maximumProduct(vector<int>& nums) {
        vector<int> max3v(3, INT_MIN);
        vector<int> min2v(2, INT_MAX);
        for(int i:nums){
            if(i>max3v[0]){
                max3v[2] = max3v[1];
                max3v[1] = max3v[0];
                max3v[0] = i;
            }else if(i>max3v[1]){
                max3v[2] = max3v[1];
                max3v[1] = i;
            }else if(i>max3v[2]){
                max3v[2] = i;
            }
            if(i<min2v[0]){
                min2v[1] = min2v[0];
                min2v[0] = i;
            }else if(i<min2v[1]){
                min2v[1] = i;
            }
        }
        return max(max3v[0]*max3v[1]*max3v[2],max3v[0]*min2v[0]*min2v[1]);
    }
};