反转二叉树

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.

<pre name="code" class="plain">/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     struct TreeNode *left; *     struct TreeNode *right; * }; */

递归版：

struct TreeNode* invertTree(struct TreeNode* root){    struct TreeNode *node;if (root == NULL)return root;node = invertTree(root->left);root->left = invertTree(root->right);root->right = node;return root;}

非递归版：

struct TreeNode* invertTree(struct TreeNode *root){struct TreeNode *node, *tmp;Stack treeStack;if (root == NULL)return root;stack_init(&treeStack, NULL);stack_push(&treeStack, root);while (treeStack.size > 0){stack_pop(&treeStack, &node);tmp = node->left;node->left = node->right;node->right = tmp;if (node->left)stack_push(&treeStack, node->left);if (node->right)stack_push(&treeStack, node->right);}stack_destory(&treeStack);return root;}