题目
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
难度
Easy
方法
采用递归的方法。如果root为空，则直接返回root; 如果root的值<L，表示root及其左子树所有节点都<L，那么需要改变root节点，从root.right中重新寻找root节点。同理，当root的值>R时，需要从root.left中重新寻找root节点。当L<=root.val<=R时，则递归处理root的左右子树。
python代码
class TreeNode(object):
def __init__(self, x):
self.val = x
self.left = None
self.right = None
class Solution(object):
def trimBST(self, root, L, R):
if root == None:
return None
if root.val < L:
return self.trimBST(root.right, L, R)
if root.val > R:
return self.trimBST(root.left, L, R)
root.left = self.trimBST(root.left, L, R)
root.right = self.trimBST(root.right, L, R)
return root
root = TreeNode(1)
root.left = TreeNode(0)
root.right = TreeNode(2)
assert Solution().trimBST(root, 3, 4) == None
root = TreeNode(3)
root.left = TreeNode(0)
root.right = TreeNode(4)
root.left.right = TreeNode(2)
root.left.right.left = TreeNode(1)
root = Solution().trimBST(root, 1, 3)
assert root.val == 3
assert root.left.val == 2
assert root.right == None
assert root.left.left.val == 1
