LeetCode_226. Invert Binary Tree

226. Invert Binary Tree

Easy

Invert a binary tree.

Example:

Input:

     4
   /   \
  2     7
 / \   / \
1   3 6   9

Output:

     4
   /   \
  7     2
 / \   / \
9   6 3   1

Trivia:
This problem was inspired by this original tweet by Max Howell:

package leetcode.easy;

public class InvertBinaryTree {
	public TreeNode invertTree1(TreeNode root) {
		if (root == null) {
			return null;
		}
		TreeNode right = invertTree1(root.right);
		TreeNode left = invertTree1(root.left);
		root.left = right;
		root.right = left;
		return root;
	}

	public TreeNode invertTree2(TreeNode root) {
		if (root == null) {
			return null;
		}
		java.util.Queue<TreeNode> queue = new java.util.LinkedList<TreeNode>();
		queue.add(root);
		while (!queue.isEmpty()) {
			TreeNode current = queue.poll();
			TreeNode temp = current.left;
			current.left = current.right;
			current.right = temp;
			if (current.left != null) {
				queue.add(current.left);
			}
			if (current.right != null) {
				queue.add(current.right);
			}
		}
		return root;
	}

	@org.junit.Test
	public void test1() {
		TreeNode tn11 = new TreeNode(4);
		TreeNode tn21 = new TreeNode(2);
		TreeNode tn22 = new TreeNode(7);
		TreeNode tn31 = new TreeNode(1);
		TreeNode tn32 = new TreeNode(3);
		TreeNode tn33 = new TreeNode(6);
		TreeNode tn34 = new TreeNode(9);
		tn11.left = tn21;
		tn11.right = tn22;
		tn21.left = tn31;
		tn21.right = tn32;
		tn22.left = tn33;
		tn22.right = tn34;
		tn31.left = null;
		tn31.right = null;
		tn32.left = null;
		tn32.right = null;
		tn33.left = null;
		tn33.right = null;
		tn34.left = null;
		tn34.right = null;
		System.out.println(tn11);
		System.out.println(invertTree1(tn11));
	}

	@org.junit.Test
	public void test2() {
		TreeNode tn11 = new TreeNode(4);
		TreeNode tn21 = new TreeNode(2);
		TreeNode tn22 = new TreeNode(7);
		TreeNode tn31 = new TreeNode(1);
		TreeNode tn32 = new TreeNode(3);
		TreeNode tn33 = new TreeNode(6);
		TreeNode tn34 = new TreeNode(9);
		tn11.left = tn21;
		tn11.right = tn22;
		tn21.left = tn31;
		tn21.right = tn32;
		tn22.left = tn33;
		tn22.right = tn34;
		tn31.left = null;
		tn31.right = null;
		tn32.left = null;
		tn32.right = null;
		tn33.left = null;
		tn33.right = null;
		tn34.left = null;
		tn34.right = null;
		System.out.println(tn11);
		System.out.println(invertTree2(tn11));
	}
}