This is the iterative version of finding the depth. The recursive version is trivial, so expect the interviewer to ask for the iterative version. I used two stacks for the dfs one and a queue for the level-order traversal one. Level order one is faster.

DFS

```
public int maxDepth(TreeNode root) {
if(root == null) {
return 0;
}
Stack<TreeNode> stack = new Stack<>();
Stack<Integer> value = new Stack<>();
stack.push(root);
value.push(1);
int max = 0;
while(!stack.isEmpty()) {
TreeNode node = stack.pop();
int temp = value.pop();
max = Math.max(temp, max);
if(node.left != null) {
stack.push(node.left);
value.push(temp+1);
}
if(node.right != null) {
stack.push(node.right);
value.push(temp+1);
}
}
return max;
}
// 7ms
```

BFS

```
public int maxDepth(TreeNode root) {
if(root == null) {
return 0;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
int count = 0;
while(!queue.isEmpty()) {
int size = queue.size();
while(size-- > 0) {
TreeNode node = queue.poll();
if(node.left != null) {
queue.offer(node.left);
}
if(node.right != null) {
queue.offer(node.right);
}
}
count++;
}
return count;
}
// 3ms
```