Iterative function to calculate power exponential and nCr implementation

nCr.cpp

ll power(ll x, ll y, ll p)
{
    ll res = 1;
    x = x % p; 
    while (y > 0)
    {
        if (y & 1)
            res = (res*x) % p;
        y = y>>1;
        x = (x*x) % p;
    }
    return res;
}
 

ll modInverse(ll n, ll p)
{
    return power(n, p-2, p);
}

ll nCr[5001][5001];
void pre()
{
	ll i, j;
 	for (i = 0; i < 5001; i++)
    {
        for (j = 0; j <=i; j++)
        {
            if (j == 0 || j == i)
                nCr[i][j] = 1;
 		    else
                nCr[i][j] = mod(nCr[i-1][j-1] + nCr[i-1][j],Mod-1);
        }
    }
}