invertBinaryTree.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode() {}
 * TreeNode(int val) { this.val = val; }
 * TreeNode(int val, TreeNode left, TreeNode right) {
 * this.val = val;
 * this.left = left;
 * this.right = right;
 * }
 * }
 */
public class Solution {
    public TreeNode invertTree(TreeNode root) {

        if (root == null) {
            return null;
        }

        final Deque<TreeNode> stack = new LinkedList<>();
        stack.push(root);

        while (!stack.isEmpty()) {
            TreeNode currentNode = stack.pop();
            TreeNode left = currentNode.left;
            currentNode.left = currentNode.right;
            currentNode.right = left;

            if (currentNode.left != null) {
                stack.push(currentNode.left);
            }
            if (currentNode.right != null) {
                stack.push(currentNode.right);
            }
        }
        return root;
    }
}