public class Solution {
    // you need treat n as an unsigned value
    public int reverseBits(int n) {
        int ret = 0;
        for(int i=0;i<32;i++){
            // n>>i & 1 找到 n中第i位的数是什么。
            ret = (ret<<1)+(n>>i &1);
        }
        return ret;
    }
}