226. Invert Binary Tree

Invert a binary tree.

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

to

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

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

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so f*** off.

Solution:
It is a tree problem, so I will give both recursion and iteration solution.

The algorithm is straightforward and I am not going to explain that.
Iteration:

  public TreeNode invertTree(TreeNode root) {  
     if(root==null) return root;  
     LinkedList<TreeNode> q=new LinkedList<TreeNode>();  
     q.offer(root);  
     while(q.size()>0){  
       TreeNode cur=q.poll();  
       TreeNode newLeft=cur.right;  
       TreeNode newRight=cur.left;  
       cur.right=newRight;  
       cur.left=newLeft;  
       if(newLeft!=null) q.offer(newLeft);  
       if(newRight!=null) q.offer(newRight);  
     }  
     return root;  
   }  

Recursion:

  public TreeNode invertTree(TreeNode root) {  
     if(root==null) return root;  
     TreeNode newLeft=invertTree(root.right);  
     TreeNode newRight=invertTree(root.left);  
     root.left=newLeft;  
     root.right=newRight;  
     return root;  
   }