“Can you write a function to traverse a binary tree in-order, and print out the value of each node as it passes?”

  4
\
5
/
6


“The example would output [4, 6, 5] since there is no left child for 4, and 6 is visited in-order before 5.

The definition of a tree node is as follows:”

function Node(val) {
this.val = val;
this.left = null;
this.right = null;
}


Another example tree:

     05
/  \
10  08
/  \
17  03


Should result in [17, 10, 3, 5, 8].

Iterating with a stack seems right for traversing a tree in order.

From a given node, we need to get to its left-most descendent node first, pushing each node we visit on the way on to the stack to look at afterwards.

function inorderTraversal(node) {
const result = [];
const stack = [];
while (true) {
// Keep descending left until there is no left child.
if (node) {
stack.push(node);
node = node.left;
continue;
}

// Take the most recent node back off the stack, take its value, and
// continue from its right child.
if (stack.length > 0) {
node = stack.pop();
result.push(node.val);
node = node.right;
continue;
}

// We've visited all nodes; finish.
break;
}
return result;
}


This should be O(n).

“Follow up: you’ll likely get the recursive solution first, could you do it iteratively?”

I actually did the iterative approach first. The recursive approach could look like this:

function inorderTraversal(node, result = []) {
if (!node) {
return result;
}
if (node.left) {
inorderTraversal(node.left, result);
}
result.push(node.val);
if (node.right) {
inorderTraversal(node.right, result);
}
return result;
}


This is a lot more compact than the iterative version, but could exhaust the execution stack for a large tree.