Question

If I have an randomly shuffled array with the numbers 1 to n, what is a good way to find that the array contains the range 1 to n (no repeats)? For example,

n = 6; [1, 3, 6, 2, 4, 5] => true
n = 6; [1, 1, 2, 4, 5, 6] => false

Answer 1

Make an array of size n, pass through your array and increment the position in the array with that as an index. If at any time the counts array has non 0 or non 1 value, you can stop. If you can’t find the index, you can stop now since you know you don’t have it.

Here’s a quick Java example. In this example, you do not need to count at the end because anything that would cause a non-1 value would cause a failure during the middle.

boolean isRange(int[] arr) {

    int[] counts = new int[arr.length];

    for(int i : arr) {
        if(i < 1 || i > arr.length) return false;
        if(counts[i - 1] != 0) return false;
        counts[i-1] = 1;
    }
    return true; // if it wasn't, we would have failed by now

}