the left part of left pointer is less than 1. [.., left)
the right part of right pointer is greater than 1. (right, ...]
the part between left pointer and i is equal to 1. [left, i)

public class sortColors {
    public static void sortColors(int[] nums) {
        if(nums==null||nums.length==0)return;
        int len = nums.length;
        int left = 0, right = len-1, i = 0;
        while (i <= right){
            System.out.println(left+"   "+i+"   "+right);
            System.out.println(Arrays.toString(nums));
            
            if (nums[i] < 1){
                swap(nums, left, i);
                left++;
                i++;
            } else if (nums[i] == 1) {
                i++;
            } else {
                swap(nums, i, right);
                right--;
            }
            System.out.println(left+"   "+i+"   "+right);
        }
        return;
    }
    
    private static void swap(int[] nums,int i,int j){
        int temp=nums[i];
        nums[i]=nums[j];
        nums[j]=temp;
    }
    public static void main(String[] args) {
        int [] nums = new int[] {2,0,0,1,1,2,2,0};
        sortColors(nums);
    }
}