Easy_Question5 : Invert Binary Tree

 Given the root of a binary tree, invert the tree, and return its root.

  Example 1:

Input: root = [4,2,7,1,3,6,9]
Output: [4,7,2,9,6,3,1]

Example 2:

Input: root = [2,1,3]
Output: [2,3,1]

Example 3:

Input: root = []
Output: []

 

Constraints:

• The number of nodes in the tree is in the range 

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) return null;
        Queue<TreeNode> queue = new 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;
    }
}

Complexity Analysis

Since each node in the tree is visited/added to the queue only once, the time complexity is $O(n)$, where $n$ is the number of nodes in the tree.

Space complexity is $O(n)$, since in the worst case, the queue will contain all nodes in one level of the binary tree. For a full binary tree, the leaf level has $\lceil \frac{n}{2}\rceil=O(n)$ leaves.