Delete the duplicate items in the ordered array -- force deduction question 26 (simple)

Remove duplicate items from an ordered array – Force to buckle 26 topic （ Simple ）
To give you one Ascending order Array of nums , Would you please In situ Delete duplicate elements , Make each element Only once , Returns the new length of the deleted array . Elemental Relative order It should be maintained Agreement .

• Because the length of an array cannot be changed in some languages , So you have to put the result in an array nums The first part of . More formally , If there is... After deleting duplicates k Elements , that nums Before k An element should hold the final result .
• Insert the final result into nums Before k Return to... After a position k .
• Don't use extra space , You must be there. In situ Modify input array And using O(1) Complete with extra space .

At first I thought of using HashMap, Simple but time-consuming 2ms（ Only more than 8% Of java Submit user ）

public  int removeDuplicates(int[] nums) {
    
        HashMap<Integer,Object> map = new HashMap<>();
        int index = 0;
        for (int i = 0; i < nums.length; i++) {
    
            if (!map.containsKey(nums[i])){
    
                map.put(nums[i],null);
                nums[index] = nums[i];
                index ++;
            }
        }
        return index;
    }

After some improvement, I thought of double pointer , Time consuming 1ms exceed 44% Of java Submit user

public int removeDuplicates(int[] nums) {
    
        int j = 0; 
        for(int i = 0;i<nums.length;i++){
    
            if(i ==0 || nums[i] != nums[i-1]){
    
                nums[j] = nums[i];
                j++;
            }
        }
        return j;
    }

Then I found that my double pointer was not standard ...

Here are the official answers , Time consuming 0ms exceed 100% Of java Submit user

public int removeDuplicates(int[] nums) {
    
        int n = nums.length;
        if (n == 0){
    
            return 0;
        }
        int fast = 1,slow = 1;
        for (int i = 1; i < n; i++) {
    
            if (nums[fast] != nums[fast-1]){
    
                nums[slow] = nums[fast];
                slow++;
            }
            fast++;
        }
        return slow;
    }

I made a dynamic diagram of the execution process according to the code

We still need the official answer ！！！