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Java Solution for LeetCode algorithm problems, continually updating

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package g0001_0100.s0094_binary_tree_inorder_traversal;

// #Easy #Top_100_Liked_Questions #Top_Interview_Questions #Depth_First_Search #Tree #Binary_Tree
// #Stack #Data_Structure_I_Day_10_Tree #Udemy_Tree_Stack_Queue
// #2022_06_21_Time_0_ms_(100.00%)_Space_42.7_MB_(9.33%)

import com_github_leetcode.TreeNode;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/*
 * 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;
 *     }
 * }
 */
public class Solution {
    public List inorderTraversal(TreeNode root) {
        if (root == null) {
            return Collections.emptyList();
        }
        List answer = new ArrayList<>();
        inorderTraversal(root, answer);
        return answer;
    }

    public void inorderTraversal(TreeNode root, List answer) {
        if (root == null) {
            return;
        }
        if (root.left != null) {
            inorderTraversal(root.left, answer);
        }
        answer.add(root.val);
        if (root.right != null) {
            inorderTraversal(root.right, answer);
        }
    }
}