Count the number of set bits (1s) in the binary representation of a non-negative integer. A set bit is a bit that has a value of 1 in the binary form of the number. For example, 5 in binary is 101, which has 2 set bits. Return 0 for input 0 and null for null input.

Examples:

Input: num = 5 Output: 2 Explanation: 5 in binary is 101, which has two 1s Input: num = 7 Output: 3 Explanation: 7 in binary is 111, which has three 1s Input: num = 8 Output: 1 Explanation: 8 in binary is 1000, which has one 1 Input: num = null Output: null Explanation: Null input returns null
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