/*

    This is an implementation that demonstrates

    how to efficiently invert a binary tree. 

    When inverting a binary tree, every left node 

    is swapped with its corresponding right node.

​

    Let n be the number of nodes of the tree and 

    h be its height.

​

    Time complexity: O(n) 

    Space complexity: O(h)

*/

​

public class InvertBinaryTree {

    private BTNode BTRoot;

​

    public InvertBinaryTree() {

        /*

         * Create tree below:

         * * 1

         * / \

         * 2 3

         * * / \

         * * 4 5

         */

        BTRoot = new BTNode(1, null, null);

        BTNode rootLeft = new BTNode(2, null, null);

        BTRoot.left = rootLeft;

        BTNode rootRight = new BTNode(3, null, null);

        BTRoot.right = rootRight;

        BTNode rootRightLeft = new BTNode(4, null, null);

        BTNode rootRightRight = new BTNode(5, null, null);

        rootRight.left = rootRightLeft;

        rootRight.right = rootRightRight;

    }

​

    public static void main(String[] args) {

        InvertBinaryTree application = new InvertBinaryTree();

        application.invertBT();

        System.out.println(application.BTRoot.val); // 1

        System.out.println(application.BTRoot.left.val); // 3

        System.out.println(application.BTRoot.right.val); // 2

        System.out.println(application.BTRoot.left.left.val); // 5

        System.out.println(application.BTRoot.left.right.val); // 4

    }

​

    private void invertBT() {

        invertBTUtil(BTRoot);

    }

​

    private void invertBTUtil(BTNode tree) {

        if (tree == null)

            return;

        // Swap current left and right

        swapLeftAndRight(tree);

        // Recursively invert left subtree

        invertBTUtil(tree.left);

        // Recursively invert right subtree

        invertBTUtil(tree.right);

    }

​

    private void swapLeftAndRight(BTNode tree) {

        BTNode temp = tree.left;

        tree.left = tree.right;

        tree.right = temp;

    }

​

    // Class representing a binary tree node

    // with pointers to value, left, and right nodes

    private class BTNode {

        int val;

        BTNode left;

        BTNode right;

​

        public BTNode(int val, BTNode left, BTNode right) {

            this.val = val;

            this.left = left;

            this.right = right;

        }

    }

}