[leetcode] 190. Reverse Bits

题目

Reverse bits of a given 32 bits unsigned integer.

Note:

• Note that in some languages, such as Java, there is no unsigned integer type. In this case, both input and output will be given as a signed integer type. They should not affect your implementation, as the integer's internal binary representation is the same, whether it is signed or unsigned.
• In Java, the compiler represents the signed integers using 2's complement notation. Therefore, in Example 2 above, the input represents the signed integer -3 and the output represents the signed integer -1073741825.

Example 1:

Input: n = 00000010100101000001111010011100
Output:    964176192 (00111001011110000010100101000000)
Explanation: The input binary string 00000010100101000001111010011100 represents the unsigned integer 43261596, so return 964176192 which its binary representation is 00111001011110000010100101000000.

Example 2:

Input: n = 11111111111111111111111111111101
Output:   3221225471 (10111111111111111111111111111111)
Explanation: The input binary string 11111111111111111111111111111101 represents the unsigned integer 4294967293, so return 3221225471 which its binary representation is 10111111111111111111111111111111.

Constraints:

• The input must be a binary string of length 32

Follow up: If this function is called many times, how would you optimize it?

思路

开辟一个长度为32的数组，用来保存二进制形式的数字，转换数字n，反转数组后还原。

代码

python版本：

class Solution:
    def reverseBits(self, n: int) -> int:
        nums = [0 for _ in range(32)]
        res = 0
        for i in range(31, -1, -1):
            if n == 0:
                break
            nums[i] = n % 2
            n //= 2
        for i in range(32):
            res += pow(2, i)*nums[i]
        return res