leetcode_357 Count Numbers with Unique Digits

i = 1 ，10 
i = 2 ，10  + 9*9 
i = 3 ，10  + 9*9  + 9*9*8 
i = 4 ，10  + 9*9  + 9*9*8  + 9*9*8*7 
......
i = n ，10  + 9*9  + 9*9*8  + 9*9*8*7  + ... + 9*9*8*7*...*(9-i+2)

//        
int countNumbersWithUniqueDigits1(int n)
{
    if (n == 0)
        return 1;
    int result = 10;
    if (n == 1)
        return result;
    int temp = 9;
    for (int i = 2; i <= n; i++)
    {
        temp = temp * (9 - i + 2);
        result += temp; 
    }
    return result;
}
//           （        ）
int countNumbersWithUniqueDigits(int n) 
{
    n = min(n, 10);
    int *p = new int[n+1];
    for (int i = 0; i <= n; i++)
    {
        p[i] = 9;
    }
    p[0] = 1;
    for (int i = 2; i <= n; i++)
    {
        for (int x = 9; x >= 9 - i + 2; x--)
        {
            p[i] *= x;
        }
    }
    int result = 0;
    for (int i = 0; i <= n; i++)
        result += p[i];
    return result;
}

//     
public int countNumbersWithUniqueDigits(int n) {
    if (n == 0)
        return 1;
    if (n == 1)
        return 10;
    int result = 10;
    int temp = 9;
    for (int i = 2; i <= n; i++) {
        temp *= (9 - i + 2);
        result += temp;
    }
    return result;
}
//           （        ）
public int countNumbersWithUniqueDigits(int n) {
    if (n == 0)
        return 1;
    if (n == 1)
        return 10;
    int [] p = new int[n + 1];
    for (int i = 0; i <= n; i++)
        p[i] = 9;
    int result = 0;
    p[0] = 1;
    for (int i = 2; i <= n; i++) {
        for (int x = 9; x >= 9 - i + 2; x--) {
            p[i] *= x;
        }
    }
    for (int i = 0; i <= n; i++) {
        result += p[i];
    }
    return result;
}