[LeetCode] 669. Trim a Binary Search Tree

Problem

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

Recursive

class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if (root == null || L > R) return null;
        if (root.val > R) return trimBST(root.left, L, R);
        if (root.val < L) return trimBST(root.right, L, R);
        if (root.val >= L && root.val <= R) {
            root.left = trimBST(root.left, L, R);
            root.right = trimBST(root.right, L, R);
        }
        return root;
    }
}

Iterative

class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if (root == null) return null;
        while (root.val < L || root.val > R) {
            if (root.val < L) {
                root = root.right;
            }
            if (root.val > R) {
                root = root.left;
            }
        }
        TreeNode dummy = root;
        while (dummy != null) {
            while (dummy.left != null && dummy.left.val < L) {
                dummy.left = dummy.left.right;
            }
            dummy = dummy.left;
        }
        
        dummy = root;
        while (dummy != null) {
            while (dummy.right != null && dummy.right.val > R) {
                dummy.right = dummy.right.left;
            }
            dummy = dummy.right;
        }
        
        return root;
    }
}
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