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Binary Tree from Array - Binary Trees | Data Structures | Repovive

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Data Structures19 sections · 729 units

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Binary Tree from Array

Level-order construction

Many problems give trees in array format: $[3, 9, 20, null, null, 15, 7]$ .

You're looking at level-order representation:

Index 0 is root
For node at index $i$ : left child at $2i+1$ , right child at $2i+2$
null indicates missing node

function arrayToTree(arr):
    if arr is empty:
        return null
    root = TreeNode(arr[0])
    queue = [root]
    i = 1
    while queue and i < arr.length:
        node = queue.dequeue()
        if arr[i] != null:
            node.left = TreeNode(arr[i])
            queue.append(node.left)
        i += 1
        if i < arr.length and arr[i] != null:
            node.right = TreeNode(arr[i])
            queue.append(node.right)
        i += 1
    return root

This BFS approach builds the tree level by level.

HannahHi! I'm Hannah. Let me know if you need help understanding anything!