检查2个二叉搜索树是否相等

Question

I'm creating a binary search tree project, and one of the questions is to create 2 trees and check if they're equal or not. When I implement the method, I keep getting firstTree and secondTree are equal . Here's the relevant code:

BstTest2 firstTree = new BstTest2();
firstTree.addNode(50, "Francisco Domingo Carlos Andres Sebastián d'Anconia");
firstTree.addNode(25, "John Galt");
firstTree.addNode(15, "Hugh Akston");
firstTree.addNode(30, "Ragnar Danneskjöld");
firstTree.addNode(85, "Hank Reardan"); //implementing add method

BstTest2 secondTree = new BstTest2();
secondTree.addNode(50, "Francisco Domingo Carlos Andres Sebastián d'Anconia");
secondTree.addNode(25, "John Galt");
secondTree.addNode(15, "Hugh Akston");
secondTree.addNode(30, "Ragnar Danneskjöld");
secondTree.addNode(75, "Midas Mulligan");
secondTree.addNode(85, "Hank Reardan");

if(firstTree.isEqual(secondTree))
{
     System.out.println("firstTree and secondTree are equal");
}
else
{
     System.out.println("firstTree and secondTree are not equal");
}

isEqual and check methods for comparing the trees

public boolean isEqual(BstTest2 tree1)
{
    return check(this.rootNode, tree1.rootNode);
}
public boolean check(Node node1, Node node2)
{
    if((node1 == null) && (node2 == null))
    {
        return true;
    } 
    else if((node1 == null) || node2 != null)
    {
        return false;
    } 
    else if((node1 != null) || node2 == null)
    {
        return false;
    } 
    else
    {
        return check(node1.leftChild, node2.leftChild) && check(node1.rightChild, node2.rightChild);
    }
}

What did I do wrong in my isEqual() and check() methods that I keep getting " firstTree and secondTree are equal" when the trees are not equal?

Answer 1

You forget to check if the value of node1 and node2 are the same.

• If they are not the same, it means these two trees are not same.
• If they are the same, we keep on checking if their left and right child are the same.

public boolean check(Node node1, Node node2)
{
    if((node1 == null) && (node2 == null))
    {
        return true;
    } 

    // If only one node is null, it means these two trees are not the same
    // These two nodes here couldn't both be null because we check this condition earlier.
    if(node1 == null || node2 == null)
    {
        return false;
    } 

    // Check if the value of node1 and node2 are the same
    if(node1.val != node2.val)
    {
        return false;
    } 

    return check(node1.leftChild, node2.leftChild) && check(node1.rightChild, node2.rightChild);
}