LeetCode 226 Invert Binary Tree - Implementation

Here is the solution:

function invertTree(root):
    if root is null:
        return null

    temp = root.left
    root.left = root.right
    root.right = temp

    invertTree(root.left)
    invertTree(root.right)

    return root

You swap the pointers with a temp variable. Then you recurse on the swapped children. The recursion propagates the inversion down the tree. Finally, you return the root.

This runs in $O(n)$ time and uses $O(h)$ space.