[LeetCode] 669. Trim a Binary Search Tree

轉自LeetCode

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>

想法如下

• 利用 recursive 來解這題

• 如果 root.val < L，代表只有 root.right 有可能 > L，所以再用 recursive 檢查

• 同理，如果 root.val > R，代表只有 root.left 有可能 < R，再用 recursive 檢查

• 如果 L <= root.val <= R，那麼對於其左右子樹，再用 recursive 檢查

code 如下

Java(參考解法)

	/**
	* 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 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;
	}
	}

view raw trimBST.java hosted with ❤ by GitHub