Code Logo

Tree Find Parent Index

Published at25 Jul 2026
Created by
Tree Traversal Easy 0 views
Like0

Given an array representing a binary tree (root at index 0, children at 2i+1 and 2i+2), find the parent index of a given node index. The root (index 0) has no parent, return -1.

For any node at index i, its parent is at index Math.floor((i-1)/2).

Example Input & Output

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

Parent of index 3 is index 1

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

Parent of index 1 is index 0

Example 3
Input
[1,2,3],2
Output
0
Explanation

Parent of index 2 is index 0

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

Root has no parent

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

Parent of index 4 is index 1

Algorithm Flow

Recommendation Algorithm Flow for Tree Find Parent Index
Recommendation Algorithm Flow for Tree Find Parent Index

Solution Approach

function solution(arr, idx) {
  if (idx <= 0 || idx >= arr.length) return -1;
  return Math.floor((idx - 1) / 2);
}

Best Answers

java
class Solution {
    public int solution(int[] nums, int i) {
        return i<=0||i>=nums.length?-1:(i-1)/2;
    }
}