669 Trim a Binary Search Tree

669. Trim a Binary Search Tree

1. Question

Given a binary search tree and the lowest and highest boundaries asLandR, 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

2. Implementation

(1) Recursion

class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if (root == null) {
            return null;
        }

        if (root.val > R) {
            return trimBST(root.left, L, R);
        }

        if (root.val < L) {
            return trimBST(root.right, L, R);
        }

        root.left = trimBST(root.left, L, R);
        root.right = trimBST(root.right, L, R);
        return root;
    }
}

3. Time & Space Complexity

时间和空间复杂度都是O(n)