Given an array of length n, find number of subsets where XOR of a subset is equal to a given number [closed]

Answer 1

From user3386109's comment, building on top of your code:

/* Warning: Untested */
int counts[1024] = {0}, ways[1024];
for(int i = 1; i <= n; ++i) counts[ arr[i] ] += 1;
for(int i = 0; i <= 1024; ++i) {
  const int z = counts[i];
  // Look for overflow here
  ways[i] = z == 0 ?
              0 :
              (int)(1U << (z-1));
}

memset(dp, 0, sizeof(dp));
dp[0][0] = 1;

for(i = 1; i <= 1024; i++){
    for(j = 0; j < 1024; j++) {
        // Check for overflow
        const int howmany = ways[i] * dp[i-1][j];
        dp[i][j] += howmany;
        dp[i][j^i] += howmany;
    }
}

cout << (dp[1024][ans]);

For calculating odd_ and even_, you can also use the following:

ⁿc₀+ⁿc₂+... = ⁿc₁+ⁿc₃... = 2^n-1

Because number of ways to select odd items = number of ways to reject odd items = number of ways to select even numbers

You can also optimize the space by keeping just 2 columns of dp arrays and reusing them as dp[i-2][x] are discarded.