06 Factorial of a Large Number

Factorials of large numbers | Practice | GeeksforGeeks

Question

Given an integer N, find its factorial. Example 1:

Input: N = 5
Output: 120
Explanation : 5! = 1*2*3*4*5 = 120

Example 2:

Input: N = 10
Output: 3628800
Explanation :
10! = 1*2*3*4*5*6*7*8*9*10 = 3628800

Expected Time Complexity: $O(N^2)$

Expected Auxilliary Space: $O(1)$​

Constraints: $1 ≤ N ≤ 1000$

Using Vector for Large Number

Logic

1. Factorial of larger numbers are unmanageable by any primitive data types.

2. Use a vector to represent a number

3. Perform multiplication just like how we used to do in school by hand

4. To multiply vector v with integer x

   1. Multiply each digit of v by x

   2. Update carry

   3. Replace each digit of v

Code

void multiply(vector<int> &res, int x) {
    long long product, carry = 0;
    for(int i = 0; i < res.size(); i++) {
        product = res[i] * x + carry;
        res[i] = product % 10;
        carry = product / 10;
    }
    
    while(carry) {
        res.push_back(carry % 10);
        carry /= 10;
    }
}

vector<int> factorial(int N){
    vector<int> res = {1};
    for(int i = 1; i <= N; i++)
        multiply(res, i);
        
    reverse(res.begin(), res.end());
    
    return res;
}

Complexity

Time Complexity: $O(n)$​

Space Complexity: $O(n)$​