README.md

Trim a Binary Search Tree

Description

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

Tags: Tree

思路

题目要求剪去搜索二叉树中不在所给区间的节点。此题主要考察搜索二叉树的特性，即在任意节点作为根节点，左侧子树的所有节点均小于根节点，右侧子树的所有节点均大于根节点。可以进行深搜，若发现节点值在所给区间左边，则可以剪去此节点的左子树，同理，节点值在区间右边则可以减掉右子树，接着将另一侧的子树根节点代替此节点即可。

Java:

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;
}

结语

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