1352. Product of the Last K Numbers

Implement the class ProductOfNumbers that supports two methods:

1. add(int num)

  • Adds the number num to the back of the current list of numbers.

2. getProduct(int k)

  • Returns the product of the last k numbers in the current list.
  • You can assume that always the current list has at least k numbers.

At any time, the product of any contiguous sequence of numbers will fit into a single 32-bit integer without overflowing.

 

Example:

Input
["ProductOfNumbers","add","add","add","add","add","getProduct","getProduct","getProduct","add","getProduct"]
[[],[3],[0],[2],[5],[4],[2],[3],[4],[8],[2]]

Output
[null,null,null,null,null,null,20,40,0,null,32]

Explanation
ProductOfNumbers productOfNumbers = new ProductOfNumbers();
productOfNumbers.add(3);        // [3]
productOfNumbers.add(0);        // [3,0]
productOfNumbers.add(2);        // [3,0,2]
productOfNumbers.add(5);        // [3,0,2,5]
productOfNumbers.add(4);        // [3,0,2,5,4]
productOfNumbers.getProduct(2); // return 20. The product of the last 2 numbers is 5 * 4 = 20
productOfNumbers.getProduct(3); // return 40. The product of the last 3 numbers is 2 * 5 * 4 = 40
productOfNumbers.getProduct(4); // return 0. The product of the last 4 numbers is 0 * 2 * 5 * 4 = 0
productOfNumbers.add(8);        // [3,0,2,5,4,8]
productOfNumbers.getProduct(2); // return 32. The product of the last 2 numbers is 4 * 8 = 32 

 

Constraints:

  • There will be at most 40000 operations considering both add and getProduct.
  • 0 <= num <= 100
  • 1 <= k <= 40000

Rust Solution

#[derive(Default)]
struct ProductOfNumbers {
    prefix: Vec<i32>,
}

impl ProductOfNumbers {
    fn new() -> Self {
        let prefix = vec![1];
        ProductOfNumbers { prefix }
    }

    fn add(&mut self, num: i32) {
        if num > 0 {
            let prev = self.prefix[self.prefix.len() - 1];
            self.prefix.push(prev * num);
        } else {
            self.prefix = vec![1];
        }
    }

    fn get_product(&self, k: i32) -> i32 {
        let k = k as usize;
        let n = self.prefix.len();
        if k >= n {
            0
        } else {
            self.prefix[n - 1] / self.prefix[n - 1 - k]
        }
    }
}

#[test]
fn test() {
    let mut obj = ProductOfNumbers::new();
    obj.add(3);
    obj.add(0);
    obj.add(2);
    obj.add(5);
    obj.add(4);
    assert_eq!(obj.get_product(2), 20);
    assert_eq!(obj.get_product(3), 40);
    assert_eq!(obj.get_product(4), 0);
    obj.add(8);
    assert_eq!(obj.get_product(2), 32);
}

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