Invert a binary tree.

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

to

4   /   \  7     2 / \   / \9   6 3   1 比较简单，二叉树里的题其实有点举一反三的意思，很多题会一道就差不多了。递归的反转左子树右子树就好了。

1 /** 2  * Definition for a binary tree node. 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     TreeNode* invertTree(TreeNode* root) {13         TreeNode* temp;14         if(root==NULL) return root;15         if(root->left==NULL&&root->right==NULL) return root;16         else{17         temp=root->left;18         root->left=invertTree(root->right);19         root->right=invertTree(temp);20         }21         22         23     }24 };