Recent Four Unique

Recent 4 Unique Elements

Problem Statement

Given a stream of 100 integers, ensure that in any window of 4 consecutive elements, all 4 values are unique (no duplicates).

All values are in the range 0, 1000.

Constraints

• Stream length: 100 elements
• Element range: 0 ≤ element ≤ 1000
• Uniqueness window: last 4 elements must all be different

Examples

Example 1:

[1, 2, 3, 4, 5, 6, 7, 8, ...]
Valid
Explanation: Each 4-element window has unique values
- Window [0:3]: [1, 2, 3, 4] → all unique
- Window [1:4]: [2, 3, 4, 5] → all unique
- Window [2:5]: [3, 4, 5, 6] → all unique

Example 2:

[1, 2, 3, 1, 5, 6, 7, 8, ...]
Invalid
Explanation: At position 3, value is 1 which equals stream[0]
- Window [0:3]: [1, 2, 3, 1] → 1 appears twice!

Example 3:

[10, 20, 30, 40, 10, 50, 60, 70, ...]
Valid
Explanation: At position 4, value is 10 which equals stream[0]
- Window [3:0]: [10, 20, 30, 40] → all unique ✓
- Window [7:4]: [10, 50, 60, 70] → all unique ✓

Example 4:

[0, 1, 2, 3, 0, 1, 2, 3, ...]
Valid
Explanation:
- Position 4: stream[4]=0 ≠ stream[3]=3, ≠ stream[2]=2, ≠ stream[1]=1 ✓
- Position 5: stream[5]=1 ≠ stream[4]=0, ≠ stream[3]=3, ≠ stream[2]=2 ✓
- Pattern repeats and satisfies the constraint