# Problem

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

Example 1:

Input: num1 = "2", num2 = "3"
Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"
Output: "56088"


Note:

1. The length of both num1 and num2 is < 110.
2. Both num1 and num2 contain only digits 0-9.
3. Both num1 and num2 do not contain any leading zero, except the number 0 itself.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

# Solution: Simulation

Simulate multiplication one digit at a time.

Time complexity: O(l1*l2)

Space complexity: O(l1 + l2)

C++

# Problem

Given a function rand7 which generates a uniform random integer in the range 1 to 7, write a function rand10 which generates a uniform random integer in the range 1 to 10.

Do NOT use system’s Math.random().

Example 1:

Input: 1
Output: [7]


Example 2:

Input: 2
Output: [8,4]


Example 3:

Input: 3
Output: [8,1,10]


Note:

1. rand7 is predefined.
2. Each testcase has one argument: n, the number of times that rand10 is called.

# Solution: Math

Time complexity: O(49/40) = O(1)

Time complexity: O(7/6 + 7 / 5) = O(1)

# Problem

Given a non-negative integer n, count all numbers with unique digits, x, where 0 ≤ x < 10n.

Example:
Given n = 2, return 91. (The answer should be the total numbers in the range of 0 ≤ x < 100, excluding [11,22,33,44,55,66,77,88,99])

# Solution: Math

f(0) = 1 (0)

f(1) = 10 (0 – 9)

f(2) = 9 * 9 (1-9 * (0 ~ 9 exclude the one from first digit))

f(3) = 9 * 9 * 8

f(4) = 9 * 9 * 8 * 7

f(x) = 0 if x >= 10

ans = sum(f[1] ~ f[n])

Time complexity: O(1)

Space complexity: O(1)

# Problem

A 3 x 3 magic square is a 3 x 3 grid filled with distinct numbers from 1 to 9 such that each row, column, and both diagonals all have the same sum.

Given an grid of integers, how many 3 x 3 “magic square” subgrids are there?  (Each subgrid is contiguous).

Example 1:

Input: [[4,3,8,4],
[9,5,1,9],
[2,7,6,2]]
Output: 1
Explanation:
The following subgrid is a 3 x 3 magic square:
438
951
276

while this one is not:
384
519
762

In total, there is only one magic square inside the given grid.


Note:

1. 1 <= grid.length <= 10
2. 1 <= grid[0].length <= 10
3. 0 <= grid[i][j] <= 15

# Solution

Time complexity: O(m*n)

Space complexity: O(1)

C++

# Problem

Find the smallest prime palindrome greater than or equal to N.

Recall that a number is prime if it’s only divisors are 1 and itself, and it is greater than 1.

For example, 2,3,5,7,11 and 13 are primes.

Recall that a number is a palindrome if it reads the same from left to right as it does from right to left.

For example, 12321 is a palindrome.

Example 1:

Input: 6
Output: 7


Example 2:

Input: 8
Output: 11


Example 3:

Input: 13
Output: 101

Note:

• 1 <= N <= 10^8
• The answer is guaranteed to exist and be less than 2 * 10^8.

# Solution: Math

All odd digits palindromes have a factor 11, they are not prime except 11 itself.

Time complexity: O(n)

Space complexity: O(1)

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