[leetcode-669-Trim a Binary Search Tree]

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] (R >= L). You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.

Example 1:

Input: 
    1
   / 
  0   2

  L = 1
  R = 2

Output: 
    1
      
       2

Example 2:

Input: 
    3
   / 
  0   4
   
    2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1

思路：

递归即可。

TreeNode* trimBST(TreeNode* root, int L, int R)
{
    if (root == NULL || L>R)return NULL;
    if (root->val == L)
    {
        root->left = NULL;
        root->right = trimBST(root->right, L + 1, R);
        return root;
    }
    else if (root->val == R)
    {
        root->right = NULL;
        root->left = trimBST(root->left, L, R - 1);
        return root;
    }
    else 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, root->val - 1);
        root->right = trimBST(root->right, root->val + 1, R);
        return root;
    }

}