Counting Bits

Given an integer n, return an array ans of length n + 1 such that for each i (0 <= i <= n), ans[i] is the number of 1's in the binary representation of i.
给定一个整数，返回一个长度的数组 n ，使得对于每个 i （ 0 <= i <= n ）， ans[i] 是的 i 二进制表示中的 1 的 ans 个 n + 1 数。

Example 1: 示例 1：

Input: n = 2 Output: [0,1,1] Explanation: 0 --> 0 1 --> 1 2 --> 10

Example 2: 示例 2：

Input: n = 5 Output: [0,1,1,2,1,2] Explanation: 0 --> 0 1 --> 1 2 --> 10 3 --> 11 4 --> 100 5 --> 101

Constraints: 约束：

0 <= n <= 105

Follow up: 跟进：

It is very easy to come up with a solution with a runtime of O(n log n). Can you do it in linear time O(n) and possibly in a single pass?
很容易提出运行时为 . O(n log n) 你能在线性时间内 O(n) 完成，也可能在一次通过中完成吗？
Can you do it without using any built-in function (i.e., like __builtin_popcount in C++)?
您可以在不使用任何内置函数的情况下做到这一点（即，就像在C++中一样 __builtin_popcount ）吗？

package org.leetcode;  
  
public class CountBits {  
    public int[] countBits(int n) {  
        int[] result = new int[n+1];  
        for(int i = 1 ; i <= n ; i++){  
            result[i] = result[i/2] + i%2;  
        }  
        return result;  
    }  
    public boolean compare(int[] result, int[] expected) {  
        if (result.length != expected.length) {  
            return false;  
        } else {  
            boolean status = true;  
            for (int i = 0; i < result.length; i++) {  
                if (result[i] != expected[i]) {  
                    status = false;  
                    break;                }  
            }  
            return status;  
        }  
    }  
  
    public static void main(String[] args) {  
        CountBits obj = new CountBits();  
        int input1 = 2;  
        int[] result1 = obj.countBits(input1);  
        int[] expected1 = {0, 1, 1};  
        // compare result1 and expected1 value  
        boolean status1 = obj.compare(result1, expected1);  
        if (status1) {  
            System.out.println("Test1 passed");  
        } else {  
            System.out.println("Test1 failed");  
        }  
  
        int input2 = 5;  
        int[] result2 = obj.countBits(input2);  
        int[] expected2 = {0, 1, 1, 2, 1, 2};  
        // compare result2 and expected2 value  
        boolean status2 = obj.compare(result2, expected2);  
        if (status2) {  
            System.out.println("Test2 passed");  
        } else {  
            System.out.println("Test2 failed");  
        }  
    }  
}