Leetcode 669 - Trim a BST

Note:

  • Look out cases in which we need to keep trimming a subtree.
  • Recursion:
    • Return value is a node, so base case is return null and result is return root.
    • Techniques we learned: root.left = recur(...), root.right = recur(...).

Given the root of a binary search tree and the lowest and highest boundaries as low and high, trim the tree so that all its elements lies in [low, high]. Trimming the tree should not change the relative structure of the elements that will remain in the tree (i.e., any node’s descendant should remain a descendant). It can be proven that there is a unique answer.

Return the root of the trimmed binary search tree. Note that the root may change depending on the given bounds.

Example:

img

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Input: root = [3,0,4,null,2,null,null,1], low = 1, high = 3
Output: [3,2,null,1]
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/**
* Definition for a binary tree node.
* function TreeNode(val, left, right) {
* this.val = (val===undefined ? 0 : val)
* this.left = (left===undefined ? null : left)
* this.right = (right===undefined ? null : right)
* }
*/
/**
* @param {TreeNode} root
* @param {number} low
* @param {number} high
* @return {TreeNode}
*/
var trimBST = function(root, low, high) {
if (!root) return null;
if (root.val < low) {
return trimBST(root.right, low, high);
}
if (root.val > high) {
return trimBST(root.left, low, high);
}
root.left = trimBST(root.left, low, high);
root.right = trimBST(root.right, low, high);
return root;
};