669. Trim a Binary Search Tree

Question:

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

Solution:

  • Method 1: divide and conquer (O(V + E), time complexity of dfs )
    • divide: we do the trim for left and right node.
    • if current node is within the range, we just return current node.
    • if current node needs to be trimmed, we need to append the right node to the very right of left subtree and return the left node of current node. ```Java /**
    • Definition for a binary tree node.
    • public class TreeNode {
    • int val;
    • TreeNode left;
    • TreeNode right;
    • TreeNode(int x) { val = x; }
    • } */ class Solution { public TreeNode trimBST(TreeNode root, int L, int R) { if(root == null) return root; TreeNode left = trimBST(root.left, L, R); root.left = left; TreeNode right = trimBST(root.right, L, R); root.right = right; if(root.val < L || root.val > R){ if(root.left == null) return root.right; else{ TreeNode node = root.left; while(node.right != null){ node = node.right; } node.right = root.right; root.right = null; return root.left; } } return root; } } ```