The iterative approach uses an explicit stack to simulate recursion:
function inorderIterative(root):
result = []
stack = []
current = root
while current or stack:
while current:
stack.push(current)
current = current.left
current = stack.pop()
result.append(current.val)
current = current.right
return result
The inner while loop pushes all left descendants. when you can't go left anymore, you pop, process, and go right.
Why learn this? Some problems require modifying traversal mid-process, which is easier iteratively. Also, understanding how recursion maps to iteration deepens your grasp of both.