What's wrong with my implementation of the Dutch National flag algorithm?

Here's the original question

Give you an array which has n integers,it has both positive and negative integers.Now you need sort this array in a special way.After that,the negative integers should in the front,and the positive integers should in the back.Also the relative position should not be changed. eg. -1 1 3 -2 2 ans: -1 -2 1 3 2.

My Java code (translated from Wikipedia's pseudocode)

package ThreewayPartition;

import java.util.Arrays;

public class ThreewayPartition {
    public void sort(int[] input, int mid) {
        int i=0;
        int j=0;
        int n = input.length - 1;

        while (j <= n) {
            if (input[j] < mid) {
                swap(input, i, j);
                i++;
                j++;
            } else if (input[j] > mid) {
                swap(input, j, n);
                n--;
            } else {
                j++;
            }
        }
    }

    private void swap(int[] arr, int j, int k) {
        int tmp = arr[j];
        arr[j] = arr[k];
        arr[k] = tmp;
    }

    public static void main(String[] args) {
        int[] input = {-1, 1, 3, -2, 2};
        ThreewayPartition twp = new ThreewayPartition();
        twp.sort(input, 0);
        System.out.println(Arrays.toString(input));
    }
}

I get this output: [-1, -2, 3, 2, 1] instead of [-1, -2, 1, 3, 2]