LeetCode Entry

1523. Count Odd Numbers in an Interval Range

07.12.2025 easy 2025 kotlin rust

Odds l..h

1523. Count Odd Numbers in an Interval Range easy blog post substack youtube

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Problem TLDR

Odds l..h #easy

Intuition

Look at total numbers count d:

• even d - just divide by 2
• odd d - divide by 2 plus look at first number if it is odd

    // 1 2      2-1+1=2,  l=o, h = e, 2/2
    // 1 2 3 4  4, l=o h=e 4/2
    // 1 2 3    3   l=o h=o   1+3/2
    // 2 3      2   l=e    2/2
    // 2 3 4 5  4   l=e    4/2
    // 3 4 5    3   l=o    1+3/2
    // 2 3 4    3   l=e    3/2
    // 3 4 5 6  4   l=o    4/2

Approach

• or brillian lee compression of this logic: (h+1)/2-l/2

Complexity

• Time complexity: \(O(1)\)

• Space complexity: \(O(1)\)

Code

// 76ms
    fun countOdds(l: Int, h: Int) =
        (h-l+1)/2 + (l%2)*((h-l+1)%2)

// 0ms
    pub fn count_odds(l: i32, h: i32) -> i32 {
       (h+1)/2 - l/2
    }