#include<math.h>
#include<stdio.h>
int missing(int arr[],int n)
{
int i=0;
while(i<n)
{
if (arr[i]-1 >= n)
i++;
else if(arr[i] < 0)
{
arr[abs(arr[i])-1]=-(arr[abs(arr[i])-1]);
i++;
}
else
{
arr[arr[i]-1]=-arr[arr[i]-1];
i++;
}
}
for(i=0;i<n;i++)
{
if(arr[i]>0)
return i+1;
}
}
int main()
{
int arr[]={7,6,1,9,10,5,2,4,8};
printf("missing number is %d\n",missing(arr,9));
return 0;
}