Invert Binary Tree

题目：

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

to

     4
   /   
  7     2
 /    / 
9   6 3   1

代码：

    public TreeNode invertTree(TreeNode root) 
    {
        TreeNode newroot = root;
        if(root==null)return newroot;
        helper(root,newroot);
        return newroot;
        
    }
    
    public TreeNode helper(TreeNode root,TreeNode newroot)
    {
        if(root==null) return newroot;
        TreeNode temp = root.left;
        root.left=root.right;
        root.right=temp;
        helper(root.left,newroot);
        helper(root.right,newroot);
        return newroot;
        
    }

解法二：

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) 
        {
            return null;
        }

        TreeNode left = root.left,right = root.right;
        root.left = invertTree(right);
        root.right = invertTree(left);
        return root;
    }

}

reference: https://leetcode.com/discuss/40001/straightforward-dfs-recursive-iterative-bfs-solutions

解法二思路如下：