Binary Tree from Array

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.