Keep at Most Two Copies in a Sorted Array Prefix
Company: Amazon
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Other
### Problem
The integer array `nums` is sorted in nondecreasing order. Modify it in place to retain at most two occurrences of each value, preserving their relative order. Return the retained length `k`; the first `k` positions must contain the resulting sorted sequence. Positions beyond that prefix are ignored.
Use constant extra memory and do not allocate a second array for the result. Preserve the original interface:
```cpp
int removeDuplicates(vector<int>& nums);
```
Return only `k`; the prefix is an observable mutation of the argument, not an additional returned array.
### Examples
```text
Input: nums = [-3,-3,-3,0,0,0,4]
Returned k: 5
Required prefix: [-3,-3,0,0,4]
```
```text
Input: nums = [6,6,6,6]
Returned k: 2
Required prefix: [6,6]
```
### Constraints
- `1 <= nums.length <= 30000`.
- Each value is between -10000 and 10000, inclusive.
- Input order is nondecreasing; retain the original relative order.
- Mutate the prefix using `O(1)` extra memory. The ignored suffix needs no special marker.
Overview: Retain at most two copies of each sorted value in the original array prefix using constant extra memory.
