LeetCode·152. Maximum Product Subarray

題目：

152. Maximum Product Subarray
Given an integer array nums, find the contiguous subarray within an array (containing at least one number) which has the largest product.

Example 1:

Input: [2,3,-2,4]
Output: 6
Explanation: [2,3] has the largest product 6.
Example 2:

Input: [-2,0,-1]
Output: 0
Explanation: The result cannot be 2, because [-2,-1] is not a subarray.

思路：

1. 動態規劃；
2. 保留上次計算的結果，包括最大值和最小值；
3. 寫出動態轉移方程（見代碼）；

代碼：

我的解法：

class Solution {
public:
    int maxProduct(vector<int>& nums) {
        if(nums.size() < 1)return 0;
        int pre_maxs = nums[0];
        int pre_mins = nums[0];
        int g_max = nums[0];

        for(int i = 1; i < nums.size(); ++i){
            int max_c = pre_maxs;
            pre_maxs = max(pre_mins*nums[i], max(nums[i], nums[i]*pre_maxs));
            pre_mins = min(max_c*nums[i], min(nums[i], nums[i]*pre_mins));
            if(g_max < pre_maxs)
                g_max = pre_maxs;
        }

        return g_max;
    }
};

最優解法：

class Solution {
public:
    int maxProduct(vector<int>& a) {

        int l = a.size();

        int maxval = a[0];
        int minval = a[0];
        int ans = a[0];

        for(int i=1;i<l;i++)
        {
            if(a[i]<0)
                swap(maxval,minval);

            maxval = max(a[i],maxval*a[i]);
            minval = min(a[i],minval*a[i]);

            ans = max(ans,maxval);
        }
        return ans;
    }
};

結果：