Duplicate numbers in array

subject ：
Find the repeated numbers in the array , At a length of n Array of nums All the numbers in 0～n-1 Within the scope of . Some numbers in the array are repeated , But I don't know how many numbers are repeated , I don't know how many times each number has been repeated . Please find any duplicate number in the array .
Formula example 1 ：
Input ：
[2, 3, 1, 0, 2, 5, 3]
Output ：
2 or 3
analysis ：
You can use sorting , After sorting the array , Compare adjacent elements in turn , See if they are equal , Equal to return to .
You can also use hash table , Store the elements in the array into the hash table , When this element is found in the hash table when saving, it returns .
Want to find the optimal solution , Be good at using the conditions in the diagram , The title says that the array length is n And all the numbers in the array are 0-n-1 Within the scope of , You can use the subscript method , Make the element equal to its corresponding subscript by constantly exchanging elements , This makes it easy to judge repeated elements . The time complexity of the optimal solution O（n）, The space complexity is O（1）.
Code ：

//  The subscript method 
    public int findRepeatNumber3(int[] nums) {
    
        for (int i = 0; i < nums.length; i++) {
    
            // use while After the exchange , The element in this position is still not in the correct position 
            while (i != nums[i]){
    
                if (nums[i] == nums[nums[i]]){
    
                    // repeat 
                    return nums[i];
                }
                int k = nums[nums[i]];
                nums[nums[i]] = nums[i];
                nums[i] = k;
            }
        }
        return -1;
    }