codeforces#P1673E. Power or XOR?
Power or XOR?
Description
The symbol $\wedge$ is quite ambiguous, especially when used without context. Sometimes it is used to denote a power ($a\wedge b = a^b$) and sometimes it is used to denote the XOR operation ($a\wedge b=a\oplus b$).
You have an ambiguous expression $E=A_1\wedge A_2\wedge A_3\wedge\ldots\wedge A_n$. You can replace each $\wedge$ symbol with either a $\texttt{Power}$ operation or a $\texttt{XOR}$ operation to get an unambiguous expression $E'$.
The value of this expression $E'$ is determined according to the following rules:
- All $\texttt{Power}$ operations are performed before any $\texttt{XOR}$ operation. In other words, the $\texttt{Power}$ operation takes precedence over $\texttt{XOR}$ operation. For example, $4\;\texttt{XOR}\;6\;\texttt{Power}\;2=4\oplus (6^2)=4\oplus 36=32$.
- Consecutive powers are calculated from left to right. For example, $2\;\texttt{Power}\;3 \;\texttt{Power}\;4 = (2^3)^4 = 8^4 = 4096$.
You are given an array $B$ of length $n$ and an integer $k$. The array $A$ is given by $A_i=2^{B_i}$ and the expression $E$ is given by $E=A_1\wedge A_2\wedge A_3\wedge\ldots\wedge A_n$. You need to find the XOR of the values of all possible unambiguous expressions $E'$ which can be obtained from $E$ and has at least $k$ $\wedge$ symbols used as $\texttt{XOR}$ operation. Since the answer can be very large, you need to find it modulo $2^{2^{20}}$. Since this number can also be very large, you need to print its binary representation without leading zeroes. If the answer is equal to $0$, print $0$.
The first line of input contains two integers $n$ and $k$ $(1\leq n\leq 2^{20}, 0\leq k < n)$.
The second line of input contains $n$ integers $B_1,B_2,\ldots,B_n$ $(1\leq B_i < 2^{20})$.
Print a single line containing a binary string without leading zeroes denoting the answer to the problem. If the answer is equal to $0$, print $0$.
Input
The first line of input contains two integers $n$ and $k$ $(1\leq n\leq 2^{20}, 0\leq k < n)$.
The second line of input contains $n$ integers $B_1,B_2,\ldots,B_n$ $(1\leq B_i < 2^{20})$.
Output
Print a single line containing a binary string without leading zeroes denoting the answer to the problem. If the answer is equal to $0$, print $0$.
Samples
3 2
3 1 2
1110
3 1
3 1 2
1010010
3 0
3 1 2
1000000000000000001010010
2 1
1 1
0
Note
For each of the testcases $1$ to $3$, $A = \{2^3,2^1,2^2\} = \{8,2,4\}$ and $E=8\wedge 2\wedge 4$.
For the first testcase, there is only one possible valid unambiguous expression $E' = 8\oplus 2\oplus 4 = 14 = (1110)_2$.
For the second testcase, there are three possible valid unambiguous expressions $E'$:
- $8\oplus 2\oplus 4 = 14$
- $8^2\oplus 4 = 64\oplus 4= 68$
- $8\oplus 2^4 = 8\oplus 16= 24$
For the third testcase, there are four possible valid unambiguous expressions $E'$:
- $8\oplus 2\oplus 4 = 14$
- $8^2\oplus 4 = 64\oplus 4= 68$
- $8\oplus 2^4 = 8\oplus 16= 24$
- $(8^2)^4 = 64^4 = 2^{24} = 16777216$
For the fourth testcase, $A=\{2,2\}$ and $E=2\wedge 2$. The only possible valid unambiguous expression $E' = 2\oplus 2 = 0 = (0)_2$.