ReverseBits

eclipse没问题，leetcode 1不能通过，超出int最大值了，但是怎么转无符号？

/*Write a function that takes an unsigned integer and returns the number of ’1' bits it has (also known as the Hamming weight).

For example, the 32-bit integer ’11' has binary representation 00000000000000000000000000001011, so the function should return 3.
*/

public static long reverseBits(int n) {
     long sum=0;
    List<Character> al=new ArrayList<Character>();
    String str = Long.toBinaryString(n);
    char[] ch = str.toCharArray();
    for(int i=0;i<32-ch.length;i++)
        al.add('0');
    for (int i = 0; i < ch.length; i++)
        al.add(ch[i]);
    System.out.println(al);
    for(int i=31;i>=0;i--) {
        if(al.get(i)=='1')
        sum+=Math.pow(2,i);}
    return sum;
    }