Given a binary search tree and the lowest and highest boundaries as L and R, trim the tree so that all its elements lies in L, R. You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.
递归的思想在二叉树的应用：
当root的值位于L和R之间，则递归修剪其左右子树，返回root。
当root的值小于L，则其左子树的值都小于L，抛弃左子树，返回修剪过的右子树。
当root的值大于R，则其右子树的值都大于R，抛弃右子树，返回修剪过的左子树。
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* struct TreeNode *left;
* struct TreeNode *right;
* };
*/
struct TreeNode* trimBST(struct TreeNode* root, int L, int R)
{
if(NULL == root)
return NULL;
if(root->val < L)
return trimBST(root->right,L,R);
else if (root->val > R)
return trimBST(root->left,L,R);
else
{
root->left = trimBST(root->left,L,R);
root->right = trimBST(root->right,L,R);
return root;
}
}