To find duplicate values in an array

Title Description:

Suppose you use an array of 1001 integers that are arbitrarily arranged, but you know that all integers are between 1 and 1000 (including 1000), and that all other numbers appear only once, except that one number appears two times. Suppose you can only handle this integer once, and use an algorithm to find the duplicate number. (no secondary storage mode)

1.sum the number,then subtract the sum from 1001*1002/2 (i.e. 1+2+3+..........+1001);

2. Using a XOR a XOR b=b; (A xor a =0)

The code is as follows:

int findx (int a[])
{
int k = a[0];
for (int i = 1; i <=; i++)
K~= A[i]~i;
return k;
}

To find duplicate values in an array