I was solving this question, namely we have given number N, which can be very big, it can have up to 100000 digits.
Now I want to know what is the most efficient way to find those digits, and I think that in big numbers I will need to delete at most 3 digits to make the number divisible by 3.
I know that number is divisible by three if the sum of its digits is divisible by three, but I can't think of how can we use this.
My idea is to brute force over the string and to check if we delete that digit is the number going to be divisible by 3, but my solution fails at complex examples. Please give me some hints.
Thanks in advance.
If the sum of the digits modulo 3 is equal to 1, you want to delete a single 1, 4, or 7. If the sum of the digits is 2, you want to delete a single 2, 5, or 8.
If that can't be done, then you have to delete two digits.
To avoid scanning the list twice, you could remember the indices of up to two digits congruent to 1, and the indices of up to two digits congruent to 2, so when you compute the final modulus you know where to look.
The number 3 has some special properties relative to a base-10 number system that you can leverage.
10 is 1 more than 9, and 9 is evenly divisible by 3, so the "1" in "10" acts as a sort of remainder from adding 1 to 9. As a result, if the sum of all digits in the number is evenly divisible by 3 then that number is also divisible by 3.
So if you begin by figuring out what the modulo is after adding all the digits, then you'll know whether the number is divisible by zero (i.e. results in a modulo zero) or not. If not, then you can subtract one digit at a time, recalculating the modulo of the resulting number until you end up with a modulo of zero.
You should check what makes a number divisible by 3. If you find it you should divide the problem into smaller problems
Related
I was struggling with a problem on Atcoder, in which we have to find minimum K for a number N such that K numbers of whose digits are either 1, 2, or 3 can sum up to N.
Through editorial, I could only understand, if
p is good if p = 10a + b
where a is either 0 or a is itself that kind of number and b is either 1, 2, or 3.
from this point onward, I'm not able under how to proceed further and what the author is meant by a good number. This problem seems really delicate but its solution is bizarre. Also, give me a better solution if possible.
problem link: https://atcoder.jp/contests/arc123/tasks/arc123_c
I am trying to understand the maths in this code that converts binary to decimal. I was wondering if anyone could break it down so that I can see the working of a conversion. Sorry if this is too newb, but I've been searching for an explanation for hours and can't find one that explains it sufficently.
I know the conversion is decimal*2 + int(digit) but I still can't break it down to understand exaclty how it's converting to decimal
binary = input('enter a number: ')
decimal = 0
for digit in binary:
decimal= decimal*2 + int(digit)
print(decimal)
Here's example with small binary number 10 (which is 2 in decimal number)
binary = 10
for digit in binary:
decimal= decimal*2 + int(digit)
For for loop will take 1 from binary number which is at first place.
digit = 1 for 1st iteration.
It will overwrite the value of decimal which is initially 0.
decimal = 0*2 + 1 = 1
For the 2nd iteration digit= 0.
It will again calculate the value of decimal like below:
decimal = 1*2 + 0 = 2
So your decimal number is 2.
You can refer this for binary to decimal conversion
The for loop and syntax are hiding a larger pattern. First, consider the same base-10 numbers we use in everyday life. One way of representing the number 237 is 200 + 30 + 7. Breaking it down further, we get 2*10^2 + 3*10^1 + 7*10^0 (note that ** is the exponent operator in Python, but ^ is used nearly everywhere else in the world).
There's this pattern of exponents and coefficients with respect to the base 10. The exponents are 2, 1, and 0 for our example, and we can represent fractions with negative exponents. The coefficients 2, 3, and 7 are the same as from the number 237 that we started with.
It winds up being the case that you can do this uniquely for any base. I.e., every real number has a unique representation in base 10, base 2, and any other base you want to work in. In base 2, the exact same pattern emerges, but all the 10s are replaced with 2s. E.g., in binary consider 101. This is the same as 1*2^2 + 0*2^1 + 1*2^0, or just 5 in base-10.
What the algorithm you have does is make that a little more efficient. It's pretty wasteful to compute 2^20, 2^19, 2^18, and so on when you're basically doing the same operations in each of those cases. With our same binary example of 101, they've re-written it as (1 *2+0)*2+1. Notice that if you distribute the second 2 into the parenthesis, you get the same representation we started with.
What if we had a larger binary number, say 11001? Well, the same trick still works. (((1 *2+1 )*2+0)*2+0)*2+1.
With that last example, what is your algorithm doing? It's first computing (1 *2+1 ). On the next loop, it takes that number and multiplies it by 2 and adds the next digit to get ((1 *2+1 )*2+0), and so on. After just two more iterations your entire decimal number has been computed.
Effectively, what this is doing is taking each binary digit and multiplying it by 2^n where n is the place of that digit, and then summing them up. The confusion comes due to this being done almost in reverse, let's step through an example:
binary = "11100"
So first it takes the digit '1' and adds it on to 0 * 2 = 0, so we
have digit = '1'.
Next take the second digit '1' and add it to 1* 2 =
2, digit = '1' + '1'*2.
Same again, with digit = '1' + '1'*2 +
'1'*2^2.
Then the 2 zeros add nothing, but double the result twice,
so finally, digit = '0' + '0'*2 + '1'*2^2 + '1'*2^3 + '1'*2^4 = 28
(I've left quotes around digits to show where they are)
As you can see, the end result in this format is a pretty simple binary to decimal conversion.
I hope this helped you understand a bit :)
I will try to explain the logic :
Consider a binary number 11001010. When looping in Python, the first digit 1 comes in first and so on.
To convert it to decimal, we will multiply it with 2^7 and do this till 0 multiplied by 2^0.
And then we will add(sum) them.
Here we are adding whenever a digit is taken and then will multiply by 2 till the end of loop. For example, 1*(2^7) is performed here as decimal=0(decimal) +1, and then multiplied by 2, 7 times. When the next digit(1) comes in the second iteration, it is added as decimal = 1(decimal) *2 + 1(digit). During the third iteration of the loop, decimal = 3(decimal)*2 + 0(digit)
3*2 = (2+1)*2 = (first_digit) 1*2*2 + (seconds_digit) 1*2.
It continues so on for all the digits.
I want to add two three digit numbers, digit by digit, and need to get value of carry after addition without using IF/ELSE and While/For. For example, if I am adding 9 and 9, I need to get the carry of these numbers but without using IF/ELSE and WHILE/FOR. Ideas?
If I've understood your question properly, I think you can achieve this with integer division.
For Example, the carry for 9 + 9 would be: (9 + 9) // 10. This would return 1.
For three digit numbers you can do: (678 + 854) // 1000.
Say I have a string whose characters are nothing but digits in [0 - 9] range. E.g: "2486". Now I want to find out all the subsequences whose sum of digits is divisible by 6. E.g: in "2486", the subsequences are - "6", "246" ( 2+ 4 + 6 = 12 is divisible by 6 ), "486" (4 + 8 + 6 = 18 is divisible by 6 ) etc. I know generating all 2^n combinations we can do this. But that's very costly. What is the most efficient way to do this?
Edit:
I found the following solution somewhere in quora.
int len,ar[MAXLEN],dp[MAXLEN][MAXN];
int fun(int idx,int m)
{
if(idx==len)
return (m==0);
if(dp[idx][m]!=-1)
return dp[idx][m];
int ans=fun(idx+1,m);
ans+=fun(idx+1,(m*10+ar[idx])%n);
return dp[idx][m]=ans;
}
int main()
{
// input len , n , array
memset(dp,-1,sizeof(dp));
printf("%d\n",fun(0,0));
return 0;
}
Can someone please explain what is the logic behind the code - 'm*10+ar[idx])%n' ? Why is m multiplied by 10 here?
Say you have a sequence of 16 digits You could generate all 216 subsequences and test them, which is 65536 operations.
Or you could take the first 8 digits and generate the 28 possible subsequences, and sort them based on the result of their sum modulo 6, and do the same for the last 8 digits. This is only 512 operations.
Then you can generate all subsequences of the original 16 digit string that are divisible by 6 by taking each subsequence of the first list with a modulo value equal to 0 (including the empty subsquence) and concatenating it with each subsequence of the last list with a modulo value equal to 0.
Then take each subsequence of the first list with a modulo value equal to 1 and concatenate it with each subsequence of the last list with a modulo value equal to 5. Then 2 with 4, 3 with 3, 4 with 2 and 5 with 1.
So after an initial cost of 512 operations you can generate just those subsequences whose sum is divisible by 6. You can apply this algorithm recursively for larger sequences.
Create an array with a 6-bit bitmap for each position in the string. Work from right to left and set the array of bitmaps so that bitmaps have bits set in the array when there is some subsequence starting from just after the array which sums up to that position in the bitmap. You can do this from right to left using the bitmap just after the current position. If you see a 3 and the bitmap just after the current position is 010001 then sums 1 and 5 are already accessible by just skipping the 3. Using the 3 sums 4 and 2 are now available, so the new bitmap is 011011.
Now do a depth first search for subsequences from left to right, with the choice at each character being either to take that character or not. As you do this keep track of the mod 6 sum of the characters taken so far. Use the bitmaps to work out whether there is a subsequence to the right of that position that, added to the sum so far, yields zero. Carry on as long as you can see that the current sum leads to a subsequence of sum zero, otherwise stop and recurse.
The first stage has cost linear in the size of the input (for fixed values of 6). The second stage has cost linear in the number of subsequences produced. In fact, if you have to actually write out the subsequences visited (E.g. by maintaining an explicit stack and writing out the contents of the stack) THAT will be the most expensive part of the program.
The worst case is of course input 000000...0000 when all 2^n subsequences are valid.
I'm pretty sure a user named, amit, recently answered a similar question for combinations rather than subsequences where the divisor is 4, although I can't find it right now. His answer was to create, in this case, five arrays (call them Array_i) in O(n) where each array contains the array elements with a modular relationship i with 6. With subsequences we also need a way to record element order. For example, in your case of 2486, our arrays could be:
Array_0 = [null,null,null,6]
Array_1 = []
Array_2 = [null,4,null,null]
Array_3 = []
Array_4 = [2,null,8,null]
Array_5 = []
Now just cross-combine the appropriate arrays, maintaining element order: Array_0, Array_2 & Array_4, Array_0 & any other combination of arrays:
6, 24, 48, 246, 486
Generally I do prime factorization and get all prime factors and I do permutation and combinations to find all factors.
For example: 1824 is the number I am trying to factors of. Now I need a no factors of 1824 within in number 300.
Is there any trick??
One trick, is not searching numbers past the square root of the number you're searching for factors. For example, to find factors from 2-300, you only really need to search from 2-ceil(sqrt(1824)), which is 2-43. Once you find a number in the 2-43 range, divide it into 1824 to check for other factors which may be above 43.
As a brute force solution , you don't need to prime factorize the number for this. You could simply check for all numbers in the range.
Let the range of numbers in which you wish to find factors be [range_start, range_end].
Simply iterate over these numbers in a loop and for each number (say x) , check if (number % x == 0 ) , if yes , then x is a factor of the number.