Formatted question description: https://leetcode.ca/all/1770.html

1770. Maximum Score from Performing Multiplication Operations

Level

Medium

Description

You are given two integer arrays nums and multipliers of size n and m respectively, where n >= m. The arrays are 1-indexed.

You begin with a score of 0. You want to perform exactly m operations. On the i-th operation (1-indexed), you will:

• Choose one integer x from either the start or the end of the array nums.
• Add multipliers[i] * x to your score.
• Remove x from the array nums.

Return the maximum score after performing m operations.

Example 1:

Input: nums = [1,2,3], multipliers = [3,2,1]

Output: 14

Explanation: An optimal solution is as follows:

• Choose from the end, [1,2,3], adding 3 * 3 = 9 to the score.
• Choose from the end, [1,2], adding 2 * 2 = 4 to the score.
• Choose from the end, [1], adding 1 * 1 = 1 to the score.

The total score is 9 + 4 + 1 = 14.

Example 2:

Input: nums = [-5,-3,-3,-2,7,1], multipliers = [-10,-5,3,4,6]

Output: 102

Explanation: An optimal solution is as follows:

• Choose from the start, [-5,-3,-3,-2,7,1], adding -5 * -10 = 50 to the score.
• Choose from the start, [-3,-3,-2,7,1], adding -3 * -5 = 15 to the score.
• Choose from the start, [-3,-2,7,1], adding -3 * 3 = -9 to the score.
• Choose from the end, [-2,7,1], adding 1 * 4 = 4 to the score.
• Choose from the end, [-2,7], adding 7 * 6 = 42 to the score.

The total score is 50 + 15 - 9 + 4 + 42 = 102.

Constraints:

• n == nums.length
• m == multipliers.length
• 1 <= m <= 10^3
• m <= n <= 10^5
• -1000 <= nums[i], multipliers[i] <= 1000

Solution

Use dynamic programming. Create a 2D array dp of m + 1 rows and n columns, where dp[i][j] represents the maximum score using the last i multiplers for the subarray starting from index j. The base case is when i = 1. The final result is dp[m][0].

To optimize memory, use a 1D array instead, which stores only the last row of the original dp array.

class Solution {
    public int maximumScore(int[] nums, int[] multipliers) {
        int n = nums.length, m = multipliers.length;
        int[] dp = new int[n];
        int minWindow = n - m + 1;
        for (int j = minWindow - 1; j < n; j++) {
            int start = j - minWindow + 1;
            dp[start] = Math.max(nums[start] * multipliers[m - 1], nums[j] * multipliers[m - 1]);
        }
        for (int i = 2; i <= m; i++) {
            int[] dpNew = new int[n];
            int window = n - m + i;
            for (int j = window - 1; j < n; j++) {
                int start = j - window + 1;
                dpNew[start] = Math.max(dp[start + 1] + nums[start] * multipliers[m - i], dp[start] + nums[j] * multipliers[m - i]);
            }
            dp = dpNew;
        }
        return dp[0];
    }
}

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