[LeetCode] Invert Binary Tree

题目：

Invert Binary Tree

解法：递归求解。交换当前节点的左节点和右节点，递归左子树，递归右子树。

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