leetcode 338 Counting Bits
z
 2019-04-09 10:36:27  

Given a non negative integer number num. For every numbers i in the range 0 ≤ i ≤ num calculate the number of 1’s in their binary representation and return them as an array.

Example 1:

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Input: 2
Output: [0,1,1]

Example 2:

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Input: 5
Output: [0,1,1,2,1,2]

Follow up:

  • It is very easy to come up with a solution with run time O(n*sizeof(integer)). But can you do it in linear time O(n) /possibly in a single pass?
  • Space complexity should be O(n).
  • Can you do it like a boss? Do it without using any builtin function like __builtin_popcount in c++ or in any other language.
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import java.util.ArrayList;

public class CountingBits {
  /**
   * Given a non negative integer number num. For every numbers i in the range 0 ≤ i ≤ num calculate the number of 1's in their binary representation and return them as an array.
   *
   * Example 1:
   *
   * Input: 2
   * Output: [0,1,1]
   * Example 2:
   *
   * Input: 5
   * Output: [0,1,1,2,1,2]
   * Follow up:
   *
   * It is very easy to come up with a solution with run time O(n*sizeof(integer)). But can you do it in linear time O(n) /possibly in a single pass?
   * Space complexity should be O(n).
   * Can you do it like a boss? Do it without using any builtin function like __builtin_popcount in c++ or in any other language.
   */
  public int[] countBits(int num) {
    int[] dp = new int[num + 1];
    dp[0] = 0;
    int width = 1;
    for (int i=1; i<=num; i++){
      dp[i] = dp[i-width] + 1;
      if (width == i){
        width *= 2;
      }
    }
    return dp;
  }
}