> This problem is paper-only - see explanation for the canonical SQL solution. (LeetCode 177 classic.)
Given `Employee(id, salary)`, return the **Nth highest distinct salary**, or null if it doesn't exist.
**For the auto-grader:** implement `nthHighestSalary(employees, n)`. Return the salary number, or null.
Example 1
Input:employees = [{id:1,salary:100},{id:2,salary:200},{id:3,salary:300}], n = 2