SetMismatch.java

public class SetMismatch {
    public int[] findErrorNums(int[] nums) {
        int i = 0;
        while(i< nums.length){
            int correct = nums[i]-1;
            if(nums[i] != nums[correct]){
                swap(nums, i,correct);
            }else {
                i++;
            }
        }
        // search for first missing number
        for (int index = 0;index< nums.length;index++){
            if (nums[index] != index + 1){
                return new int[] {nums[index], index+1};
            }
        }
        return new int[] {};
    }
    static void swap(int[] arr, int first, int second){
        int temp= arr[first];
        arr[first] = arr[second];
        arr[second] = temp;
    }
}