152. Maximum Product Subarray

Find the contiguous subarray within an array (containing at least one number) which has the largest product.

For example, given the array[2,3,-2,4], the contiguous subarray[2,3]has the largest product =6.

Thoughts:

Since two negative numbers multiply could result in positive, here we need to have two arrays, dmax, dmin to keep track of max product and min product so far

Code

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

        for(int i  = 1, dmin = nums[0], dmax = nums[0]; i < n; i++){
            if(nums[i] < 0)
                swap(dmin, dmax);

            dmin = min(nums[i], dmin*nums[i]);
            dmax = max(nums[i], dmax*nums[i]);

            maxP = maxP > dmax? maxP: dmax;
        }

        return maxP;
    }
};

class Solution {
    public int maxProduct(int[] nums) {
        if(nums == null && nums.length == 0) return 0;
        // initial value
        int max = nums[0], min = nums[0], ans = nums[0];
        for (int i = 1; i < nums.length; i++){
            int prevMin = min, prevMax = max;
            max = Math.max(nums[i], Math.max(prevMin * nums[i], prevMax * nums[i]));
            min = Math.min(nums[i], Math.min(prevMin * nums[i], prevMax * nums[i]));
            // update results
            ans = Math.max(ans, max);
        }
        return ans;
    }
}