Algo to find duplicates in an array having elements in the range [1,n]

Algorithm 1

input[n]  //input array of size n, containing n  numbers

int output[n]; // Supporting array for our algo, initialized to 0
for(int i=0;i<n;i++)
output[input[i]]++;  //using elements of input array as index to create the o/p array

for(int i=0;i<n;i++)
{
if(output[i]==0)  // If the ith element is zero, it means i wasn’t present in input array

cout<<i; // display i
}

Algorithm 2 << My favourite

traverse the list for i= 1st to n+2 elements
{
check for sign of A[abs(A[i])] ;
if positive then
   make it negative by   A[abs(A[i])]=-A[abs(A[i])];
else  // i.e., A[abs(A[i])] is negative
   this   element (ith element of list) is a repetition
}