LeetCode in Net

190. Reverse Bits

Easy

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?

Solution

public class Solution {
    // you need treat n as an unsigned value
    public uint reverseBits(uint n) {
        uint ret = 0;
        // because there are 32 bits in total
        for (int i = 0; i < 32; i++) {
            ret = ret << 1;
            // If the bit is 1 we OR it with 1, ie add 1
            if ((n & 1) > 0) {
                ret = ret | 1;
            }
            n = n >> 1;
        }
        return ret;
    }
}

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