Find a duplicate in array of integers

Question

This was an interview question.

I was given an array of n+1 integers from the range [1,n]. The property of the array is that it has k (k&

Answer 1

Here is a possible implementation:

function checkDuplicate(arr) {
  console.log(arr.join(", "));
  let  len = arr.length
      ,pos = 0
      ,done = 0
      ,cur = arr[0]
      ;
  while (done < len) {
    if (pos === cur) {
      cur = arr[++pos];
    } else {
      pos = cur;
      if (arr[pos] === cur) {
        console.log(`> duplicate is ${cur}`);
        return cur;
      }
      cur = arr[pos];
    }
    done++;
  }
  console.log("> no duplicate");
  return -1;
}

for (t of [
     [0, 1, 2, 3]
    ,[0, 1, 2, 1]
    ,[1, 0, 2, 3]
    ,[1, 1, 0, 2, 4]
  ]) checkDuplicate(t);

It is basically the solution proposed by @maraca (typed too slowly!) It has constant space requirements (for the local variables), but apart from that only uses the original array for its storage. It should be O(n) in the worst case, because as soon as a duplicate is found, the process terminates.