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.

/**
 * 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;
}