[LeetCode-JAVA] Invert Binary Tree

题目：

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 fuck off.

题意：反转一个二叉树，加点题外话，很简单的一个题，不过有关于这道题的小故事，点击链接即可。

思路1：利用递归

代码：

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null || (root.left == null && root.right == null))
            return root;
        
        TreeNode temp = root.right;
        root.right = invertTree(root.left);
        root.left = invertTree(temp);
        
        return root;
    }
}

思路2：非递归，利用stack

代码2：

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return null; 
        Stack<TreeNode> stack = new Stack<TreeNode>();  
        stack.push(root);  
        
        while(!stack.isEmpty()){  
            TreeNode cur = stack.pop();
            TreeNode temp = cur.left;
            cur.left = cur.right;
            cur.right = temp; 
            
            if(cur.left != null) stack.push(cur.left);  
            if(cur.right != null) stack.push(cur.right);  
        }  
        return root;
    }
}