[Solved] Why is the time complexity of the sum of the values of nodes in a balanced BST O(n)?

vhvg vhcc Asks: Why is the time complexity of the sum of the values of nodes in a balanced BST O(n)?
I’m trying to learn about Big O notation and I came across a problem. As in this code we’re trying to find the sum of the values of the nodes. And as there are 2 calls to sum function so each call will have twice as many calls than before. So why the runtime is O(n) and not O(2^n).

Code:
int sum(Node node) {
if(node==null)
return 0;
}
return sum(node.left) + node.value + sum(node.right);
}

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