872. Leaf-Similar Trees

Hello, reader 👋🏽 ! Welcome to day 68 of the series on Problem Solving. Through this series, I aim to pick up at least one question everyday and share my approach for solving it.

Today, I will be picking up LeetCode's daily challenge problem: 872. Leaf-Similar Trees.

🤔 Problem Statement

• Consider all the leaves of a binary tree, from left to right order, the values of those leaves form a leaf value sequence*.*

• Two binary trees are considered leaf-similar if their leaf value sequence is the same.

• Return true if and only if the two given trees with head nodes root1 and root2 are leaf-similar.

• E.g:

  • root1 = [3,5,1,6,2,9,8,null,null,7,4], root2 = [3,5,1,6,7,4,2,null,null,null,null,null,null,9,8] => true

  • root1 = [1,2,3], root2 = [1,3,2] => false

💬 Thought Process - Generate and Equate List of Leaf Nodes

• The most intuitive way to solve this problem is to separately traverse each tree and add the values of all the leaf nodes to a list.

• Once the traversal of both the trees are complete, we can find if every leaf node in both the trees have the same values.

• For both the trees, we add the lead nodes values to separate lists.

👩🏽‍💻 Solution - Generate and Equate List of Leaf Nodes

• Below is the code for the approach for generating the list of leaf nodes from both the trees and equating them.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {

    public boolean leafSimilar(TreeNode root1, TreeNode root2) {
        List<Integer> leafValues1 = new ArrayList(), 
                      leafValues2 = new ArrayList();

        traverseNodes(root1, leafValues1);
        traverseNodes(root2, leafValues2);

        if(leafValues1.size() != leafValues2.size()) {
            return false;
        }
        for(int i = 0; i < leafValues1.size(); i++) {
            if(leafValues1.get(i) != leafValues2.get(i)) {
                return false;
            }
        }

        return true;
    }

    private void traverseNodes(TreeNode root, 
                               List<Integer> leafValues) 
    {
        if(root == null) return;

        if(root.right == null && root.left == null) {
            leafValues.add(root.val);
            return;
        }

        traverseNodes(root.left, leafValues);
        traverseNodes(root.right, leafValues);
    }
}

Time Complexity: O(n + m)
  - n = number of nodes in the tree 1
  - m = number of nodes in the tree 2
Space Complexity: O(n + m)
  - n = number of nodes in the tree 1
  - m = number of nodes in the tree 2
    => space to store the leaf nodes in a list

• You can find the link to the GitHub repo for this question here: 872. Leaf-Similar Trees.

Conclusion

That's a wrap for today's problem. If you liked my explanation then please do drop a like/ comment. Also, please correct me if I've made any mistakes or if you want me to improve something!

Thank you for reading!