226. Invert Binary Tree

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

 

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

Solution one:

Recursion:  

class Solution {
    public TreeNode invertTree(TreeNode root) {
        
        return invertTreeHelper(root);
    }

        if(root == null) return null;
        TreeNode left = invertTreeHelper(root.left);
        TreeNode right = invertTreeHelper(root.right); 
        TreeNode temp = left;
        root.left = right;
        root.right = temp;
        return root;
        
        
    }
}

Iterative: 

class Solution {
    public TreeNode invertTree(TreeNode root) {
        
        return invertTreeHelper(root);
    }
    public TreeNode invertTreeHelper(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;

    }
}

 Hints:

1: Understand Recursion

2: Know the logic of Iterative, If current treenode is not null, queue.add(TreeNode)

3: LinkedList implement the interface of queue