数组中范围产品的Javascript程序

给定一个大小为 N 的数组 A[]。解决 Q 个查询。在模 P 下找到范围 [L, R] 中的产品（P 是素数）。

例子：

Input : A[] = {1, 2, 3, 4, 5, 6} 
          L = 2, R = 5, P = 229
Output : 120

Input : A[] = {1, 2, 3, 4, 5, 6},
         L = 2, R = 5, P = 113
Output : 7 

蛮力
对于每个查询，遍历 [L, R] 范围内的每个元素并计算模 P 下的乘积。这将在 O(N) 中回答每个查询。

Javascript

      
    // Product in range Queries in O(N)
      
    // Function to calculate 
    // Product in the given range.
    function calculateProduct(A, L, R, P)
    {
            
        // As our array is 0 based 
        // as and L and R are given 
        // as 1 based index.
        L = L - 1;
        R = R - 1;
        
        let ans = 1;
        for (let i = L; i <= R; i++)
        {
            ans = ans * A[i];
            ans = ans % P;
        }
        
        return ans;
    }
      
    let A = [ 1, 2, 3, 4, 5, 6 ];
    let P = 229;
    let L = 2, R = 5;
    document.write(calculateProduct(A, L, R, P) + "</br>");
  
    L = 1; 
    R = 3;
    document.write(calculateProduct(A, L, R, P) + "</br>");
          

输出 ：

120
6

高效使用模乘逆：
由于 P 是素数，我们可以使用模乘逆。使用动态规划，我们可以计算模 P 下的前积数组，使得索引 i 处的值包含 [0, i] 范围内的乘积。类似地，我们可以计算模 P 下的预逆积。现在每个查询都可以在 O(1) 中得到回答。
逆积数组包含索引 i 处 [0, i] 范围内的逆积。因此，对于查询 [L, R]，答案将是 Product[R]*InverseProduct[L-1]
注意：我们不能将答案计算为 Product[R]/Product[L-1]，因为乘积是在模 P 下计算的。如果我们不计算模 P 下的乘积，总是有溢出的可能性。

Javascript

    // Javascript program to find Product
    // in range Queries in O(1)
      
    let MAX = 100;
    let pre_product = new Array(MAX);
    let inverse_product = new Array(MAX);
       
    // Returns modulo inverse of
    // a with respect to m using
    // extended Euclid Algorithm
    // Assumption: a and m are
    // coprimes, i.e., gcd(a, m) = 1
    function modInverse(a, m)
    {
        let m0 = m, t, q;
        let x0 = 0, x1 = 1;
       
        if (m == 1)
            return 0;
       
        while (a > 1)
        {
       
            // q is quotient
            q = parseInt(a / m, 10);
            t = m;
       
            // m is remainder now, process
            // same as Euclid's algo
            m = a % m;
            a = t;
            t = x0;
            x0 = x1 - q * x0;
            x1 = t;
        }
       
        // Make x1 positive
        if (x1 < 0)
            x1 += m0;
       
        return x1;
    }
       
    // calculating pre_product array
    function calculate_Pre_Product(A, N, P)
    {
        pre_product[0] = A[0];
       
        for (let i = 1; i < N; i++)
        {
            pre_product[i] =
                pre_product[i - 1] *
                               A[i];
                                   
            pre_product[i] =
                pre_product[i] % P;
        }
    }
       
    // Cacluating inverse_product
    // array.
    function calculate_inverse_product(A, N, P)
    {
        inverse_product[0] =
                modInverse(pre_product[0], P);
       
        for (let i = 1; i < N; i++)
            inverse_product[i] =
                modInverse(pre_product[i], P);
    }
       
    // Function to calculate Product
    // in the given range.
    function calculateProduct(A, L, R, P)
    {
           
        // As our array is 0 based as
        // and L and R are given as 1
        // based index.
        L = L - 1;
        R = R - 1;
        let ans;
       
        if (L == 0)
            ans = pre_product[R];
        else
            ans = pre_product[R] *
                  inverse_product[L - 1];
       
        return ans;
    }
           
    // Array
    let A = [ 1, 2, 3, 4, 5, 6 ];
  
    // Prime P
    let P = 113;
  
    // Calculating PreProduct and
    // InverseProduct
    calculate_Pre_Product(A, A.length, P);
  
    calculate_inverse_product(A, A.length, P);
  
    // Range [L, R] in 1 base index
    let L = 2, R = 5;
    document.write(calculateProduct(A, L, R, P) + "</br>");
  
    L = 1;
    R = 3;
    document.write(calculateProduct(A, L, R, P));
        

输出 ：

7
6

有关更多详细信息，请参阅有关数组范围产品的完整文章！