public class DayCode {    public static void main(String[] args) {        int num = 10;        int[] ans = new DayCode().countBits(num);        System.out.println("ans is " + Arrays.toString(ans));    }        /**     * https://leetcode-cn.com/problems/counting-bits/     * @param num     * @return     */    public int[] countBits(int num) {        int[] bits = new int[num + 1];        for (int i = 0; i <= num; i++) {            bits[i] = countOne(i);        }        return bits;    }
    public int countOne(int i) {        int cnt = 0;        while (i > 0) {            i &= (i - 1);            cnt++;        }        return cnt;    }}

/** Returns the number of one-bits in the two's complement binary representation of the specified {@code int} value.  This function is sometimes referred to as the <i>population count</i>.  @param i the value whose bits are to be counted @return the number of one-bits in the two's complement binary representation of the specified {@code int} value. @since 1.5*/public static int bitCount(int i) {    // HD, Figure 5-2    i = i - ((i >>> 1) & 0x55555555);    i = (i & 0x33333333) + ((i >>> 2) & 0x33333333);    i = (i + (i >>> 4)) & 0x0f0f0f0f;    i = i + (i >>> 8);    i = i + (i >>> 16);    return i & 0x3f;}