31.leetcode题目226： Invert Binary Tree

题目：

Invert a binary tree.

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

to

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

分析：递归调用

class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(root==NULL)
        return root;
        TreeNode* temp=root->left;
        root->left=root->right;
        root->right=temp;
        root->left=invertTree(root->left);
        root->right=invertTree(root->right);
        return root;
    }
};

Invert a binary tree.

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

to

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