Leetcode 872. Leaf-Similar Trees

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

For example, in the given tree above, the leaf value sequence is (6, 7, 4, 9, 8).

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.

Note:

• Both of the given trees will have between 1 and 100 nodes.

Code (Java):

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean leafSimilar(TreeNode root1, TreeNode root2) {
        if (root1 == null) {
            return root2 == null;
        }
        
        if (root2 == null) {
            return root1 == null;
        }
        
        List<Integer> sequence1 = findLeafValueSequence(root1);
        List<Integer> sequence2 = findLeafValueSequence(root2);
        
        if (sequence1.size() != sequence2.size()) {
            return false;
        }
        
        for (int i = 0; i < sequence1.size(); i++) {
            int num1 = sequence1.get(i);
            int num2 = sequence2.get(i);
            
            if (num1 != num2) {
                return false;
            }
        }
        
        return true;
    }
    
    private List<Integer> findLeafValueSequence(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        
        findLeafValueSequenceHelper(root, ans);
        
        return ans;
    }
    
    private void findLeafValueSequenceHelper(TreeNode root, List<Integer> ans) {
        if (root == null) {
            return;
        }
        
        if (root.left == null && root.right == null) {
            ans.add(root.val);
            return;
        }
        
        findLeafValueSequenceHelper(root.left, ans);
        findLeafValueSequenceHelper(root.right, ans);
    }
}