Given a row number (0-indexed), generate that row of Pascal Triangle. Row 0 is [1], row 1 is [1, 1], row 2 is [1, 2, 1], row 3 is [1, 3, 3, 1], and so on. Each element is the sum of the two elements directly above it in the previous row. Return null for null input and an empty list for negative row numbers.
Examples:
Input: rowNum = 0
Output: [1]
Explanation: Row 0 is the apex of the triangle, always containing just [1]
Input: rowNum = 4
Output: [1, 4, 6, 4, 1]
Explanation: Row 4 has 5 elements built from row 3 ([1, 3, 3, 1])
Input: rowNum = 1
Output: [1, 1]
Explanation: Row 1 always contains two 1s
Input: rowNum = null
Output: null
Explanation: Null input returns null
Input: rowNum = -1
Output: []
Explanation: Negative row numbers return an empty list
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