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.

Solution 1:

class Solution {

    public TreeNode trimBST(TreeNode root, int L, int R) {

        if (root == null) {

           return null;

        }

        if (root.val < L) {

          return trimBST(root.right, L, R);

        }

        if (root.val > R) {

          return trimBST(root.left, L, R);

        }

        root.left = trimBST(root.left, L, R);

        root.right = trimBST(root.right, L, R);

        return root;

    }

}

Solution 2:

class Solution {

    public TreeNode trimBST(TreeNode root, int L, int R) {

        if (root == null) {

            return root;

        }

        while (root.val < L || root.val > R) {

            if (root.val < L) {

                root = root.right;

            }

            if (root.val > R) {

                root = root.left;

            }

        }

        TreeNode dummy = root;

        // Remove the invalid nodes from left subtree.

        while (dummy != null) {

            while (dummy.left != null && dummy.left.val < L) {

                dummy.left = dummy.left.right; 

                // If the left child is smaller than L, then we just keep the right subtree of it. 

            }

            dummy = dummy.left;

        }

        dummy = root;

        // Remove the invalid nodes from right subtree

        while (dummy != null) {

            while (dummy.right != null && dummy.right.val > R) {

                dummy.right = dummy.right.left;

                // If the right child is biggrt than R, then we just keep the left subtree of it. 

            }

            dummy = dummy.right;

        }

        return root;

    }

}