你能在二叉搜索树的构造中发现一个错误吗？

Question

I am going through AlgoExpert and learning about Binary Search trees and their construction for the first time. The below implementation seems to be working locally for me as expected but I am getting errors for most test cases on AlgoExpert. This is my implementation:

class BST {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }

  insert(value) {

    const searchTree = (node) => {
      if (value < node.value) {
        if (!node.left) {
          node.left = new BST(value);
        } else {
          searchTree(node.left);
        }
      } else {
        if (!node.right) {
          node.right = new BST(value);
        } else {
          searchTree(node.right);
        }
      }
    };
    searchTree(this);
    // Write your code here.
    // Do not edit the return statement of this method.
    return this;
  }

  contains(value) {
    let currentNode = this;
    while (currentNode) {
      if (currentNode.value === value) {
        return true;
      }
      if (value < currentNode.value) {
        currentNode = currentNode.left;
      } else {
        currentNode = currentNode.right;
      }
    }
    return false;
    // Write your code here.
  }

  min() {
    let currentNode = this;
    while (currentNode.left) {
      currentNode = currentNode.left;
    }
    return {
      value: currentNode.value,
      node: currentNode
    };
  }

  remove(value) {
    let nodeToBeRemoved = this;
    while (nodeToBeRemoved.value !== value) {
      if (value < nodeToBeRemoved.value) {
        nodeToBeRemoved = nodeToBeRemoved.left;
      } else {
        nodeToBeRemoved = nodeToBeRemoved.right;
      }
    }
    if (!nodeToBeRemoved.right) {
      nodeToBeRemoved.value = null;
    } else {
      const {
        value: minValue,
        node
      } = nodeToBeRemoved.right.min();
      node.value = null;
      nodeToBeRemoved.value = minValue;
    }
    // Write your code here.
    // Do not edit the return statement of this method.
    return this;
  }
}

Can you see any mistake that could cause this error on Algo Expert? I don't see where the mistake is. This is the error I am getting:

 Cannot read property 'toString' of null TypeError: Cannot read property 'toString' of null at constructBinaryTreeWithUniqueIds at constructBinaryTreeWithUniqueIds...

Answer 1

The remove method has some issues:

• After the first while loop, nodeToBeRemoved could be null . In that case the next if statement will make an invalid reference with nodeToBeRemoved.right .
• nodeToBeRemoved.value = null does not remove a node. It just modifies the value of a node to null . But it is still part of the tree, and could even have a left child.
• The same is true for node.value = null . This remains part of the tree and could have a right child.

remove(value) {
    let nodeToBeRemoved = this;
    if (value < this.value) {
        if (this.left) this.left = this.left.remove(value);
    } else if (value > this.value) {
        if (this.right) this.right = this.right.remove(value);
    } else if (!this.right) {
        return this.left;
    } else if (!this.left) {
        return this.right;
    } else {
        const minValue = this.right.min().value;
        this.right.remove(minValue); // use recursion to delete that min-node
        this.value = minValue;
    }
    return this;
}