reverse bit 32 unsigned integer - with issue - integer 32 bit - first bit 1 goes to negative number

reverseBit32UnsignedIntegerC.java

public class Solution {
	public long reverse(long a) {
	    long ret = 0; 
	    int index=0; 
	    long power = 31; 
	    while(a > 0 && index < 32)
	    {
	        int val = (int)(a &1); 
	        
	        if(val == 1)
	            ret += 1 << power; 
	            
	        a = a >>1; // right shift
	        index++; 
	        power--; 
	    }
	    
	    return ret; 
	}
}

https://codelab.interviewbit.com/problems/revbits/
Sorry, wrong answer. Your program's output doesn't match the expected output. You can try testing your code with custom input and try putting debug statements in your code.
Your submission failed for the following input:
A : 3
Your function returned the following :
-1073741824
The expected returned value :
3221225472