Invert a binary tree.

     4
   /   \   2     7
 / \   / \ 1   3 6   9

to

     4
   /   \   7     2
 / \   / \ 9   6 3   1

思路：
递归解决，倒置左子树，倒置右子树，交换左右子节点指针.

/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(root==NULL)
        {
            return root;
        }
        else if(root->left!=NULL||root->right!=NULL)
        {
            invertTree(root->left);       //转置左子树
            invertTree(root->right);      //转置右子树
            TreeNode* p = root->left;     //交换左右子树指针
            root->left=root->right;
            root->right=p;
        }
        return root;

    }
};