152 Maximum Product Subarray

Leetcode

Given an integer array nums, find a contiguous non-empty subarray within the array that has the largest product, and return the product.

The test cases are generated so that the answer will fit in a 32-bit integer.

A subarray is a contiguous subsequence of the array.

Example 1:

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

Example 2:

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

---

使用动态规划，每遍历到一个点时，记录当前位置的最大值和最小值，到新的位置，新的位置的值，以及新的位置的值与前一个节点的最大值和最小值的乘积，这三个数中的最大值和最小值是新位置的最大值和最小值。记录其中的最大值。

class Solution {
    public int maxProduct(int[] nums) {
        int max=nums[0];
        int big=max, small=max;
        for(int i=1;i<nums.length;++i){
            int cur=nums[i];
            int next1=cur*big;
            int next2=cur*small;
            big=max(cur, next1, next2);
            small=min(cur, next1, next2);
            if(big>max) max=big;
        }
        return max;
    }
    
    private int max(int i1, int i2, int i3){
        if(i1>i2){
            if(i1>i3) return i1;
        } else {
            if(i2>i3) return i2;
        }
        return i3;
    }
    
    private int min(int i1, int i2, int i3){
        if(i1<i2){
            if(i1<i3) return i1;
        } else {
            if(i2<i3) return i2;
        }
        return i3;
    }
}