Invert Binary Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* invertTree(struct TreeNode* root) {
    struct TreeNode *left, *right;
    if(root == NULL)
        return NULL;
    else if(root->left == NULL && root->right == NULL)
        return root;
    else
        {
            left = invertTree(root->right);    //这里必须保存到left这个临时变量中
            right = invertTree(root->left);    //否则下一次的翻转会失效
            root->left = left;
            root->right = right;
            return root;
        }
}