At my first implementation I got WA, so why ?

The problem is that we need to make it clear how to use the count and what the count means.

For every new number, count=1 means it appears for 1 time, if every number can appear for no more than

2 times, so we can check

```
if(count<2)
```

Here is the AC code:

```
class Solution {
public:
int removeDuplicates(vector<int>& nums) {
int len=nums.size();
if(len<=2) return len;
int count=1;
int start=0, end=1;
while(end<len){
if(nums[end]!=nums[start]){
nums[++start]=nums[end++];
count=1;
}
else{
if(count<2){
nums[++start]=nums[end];
count++;
}
end++;
}
}
return start+1;
}
};
```