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338. Counting Bits 👍

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  vector<int> countBits(int n) {
    // f(i) := i's number of 1s in bitmask
    // f(i) = f(i / 2) + i % 2
    vector<int> ans(n + 1);

    for (int i = 1; i <= n; ++i)
      ans[i] = ans[i / 2] + (i & 1);

    return ans;
  }
};

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class Solution {
  public int[] countBits(int n) {
    // f(i) := i's number of 1s in bitmask
    // f(i) = f(i / 2) + i % 2
    int[] ans = new int[n + 1];

    for (int i = 1; i <= n; ++i)
      ans[i] = ans[i / 2] + (i % 2);

    return ans;
  }
}

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class Solution:
  def countBits(self, n: int) -> list[int]:
    # f(i) := i's number of 1s in bitmask
    # f(i) = f(i / 2) + i % 2
    ans = [0] * (n + 1)

    for i in range(1, n + 1):
      ans[i] = ans[i // 2] + (i & 1)

    return ans