7.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 awhiteboard so fuck off. 

分析可知：这道题目是要求实现二叉树的反转。

所以在实现的过程中我采用的是递归的做法。

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

public TreeNode invertTree(TreeNode root) {if (root == null || root.left == null && root.right == null) {/*在这个地方加上判断根节点的左右孩子是否为空，则少去了对整体二叉树中每个叶子节点再进行递归*/return root;}TreeNode temp = root.left;root.left = root.right;root.right = temp;invertTree(root.left);invertTree(root.right);return root;}

之前做的Maximum Depth of Binary Tree题目和这Invert Binary Tree题目、Same Tree题目都是运用了递归的思想。