I am solving LeetCode 190. Reverse Bits problem:

Reverse bits of a given 32 bits unsigned integer.

Sample test cases:

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.

My solution:

public class Solution {
    // you need treat n as an unsigned value
    public int reverseBits(int n) {  
      int last = 0,i,sign = 0;    
      int sum = 0; 
   
      for (i = 0; i < 32; i  ){
        sum = sum<<1   n&1;
        n = n>>1;  
      }

      return sum;
    }
}

Here is what I am doing in my solution: right shifting the existing sum and adding the anding with n to it. Finally left shifting the n. These both operations as a whole are reversing the bits. But it doesn't pass the test cases:

Input               00000010100101000001111010011100
Output           0 (00000000000000000000000000000000)
Expected 964176192 (00111001011110000010100101000000)

What am I doing wrong?

CodePudding user response：

Be careful with operations priority, you are doing wrong in sum = sum<<1 n&1 line, it should be

 sum = (sum << 1)   (n & 1);

note parenthesis which are required here; your original code is equal to sum = sum << (1 n) & 1;. Let me brush up your code, you don't want last, sign etc.:

public class Solution {
    public int reverseBits(int n) {
        int sum = 0; 
   
        for (int i = 0; i < 32;   i, n >>= 1)
            sum = (sum << 1)   (n & 1);

        return sum;
    }
}