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Tree Count Nodes at Level

Published at25 Jul 2026
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Binary Tree Easy 0 views
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Given an array representing a binary tree and a level number, count how many nodes exist at that level. Level 0 is the root. In array representation, level n has indices from 2^n-1 to min(2^(n+1)-2, len-1).

Example Input & Output

Example 1
Input
[1,2,3,4,5,6,7],0
Output
1
Explanation

Root level has 1 node

Example 2
Input
[1,2,3,4,5,6,7],2
Output
4
Explanation

Level 2 has 4 nodes (indices 3-6)

Example 3
Input
[1,2,3,4,5,6,7],1
Output
2
Explanation

Level 1 has 2 nodes (indices 1,2)

Example 4
Input
[],0
Output
0
Explanation

Empty

Example 5
Input
[1,2,3,4,5],2
Output
2
Explanation

Level 2 has 2 nodes (indices 3,4)

Algorithm Flow

Recommendation Algorithm Flow for Tree Count Nodes at Level
Recommendation Algorithm Flow for Tree Count Nodes at Level

Solution Approach

function solution(arr, level) {
  if (arr.length === 0) return 0;
  var start = Math.pow(2, level) - 1;
  var end = Math.pow(2, level + 1) - 2;
  if (start >= arr.length) return 0;
  return Math.min(end, arr.length - 1) - start + 1;
}

Best Answers

java
class Solution {
    public int solution(int[] nums, int l) {
        if(nums.length==0)return 0;int s=(1<<l)-1,e=(1<<(l+1))-2;
        if(s>=nums.length)return 0;
        return Math.min(e,nums.length-1)-s+1;
    }
}