https://leetcode.com/problems/binary-tree-preorder-traversal/description/

SimpleBinaryPreOrderTraversal.java

import java.util.ArrayList;
import java.util.List;

class SimpleBinaryPreOrderTraversal {

    List<Integer> values = new ArrayList<>();
    public static void main(String[] args) {
        TreeNode node4 = new TreeNode(4);
        TreeNode node5 = new TreeNode(5);
        TreeNode node6 = new TreeNode(6);
        TreeNode node7 = new TreeNode(7);
        TreeNode node3 = new TreeNode(3);
        TreeNode node2 = new TreeNode(2,node3, null);
        TreeNode node1 = new TreeNode(1);
        System.out.println(new SimpleBinaryPreOrderTraversal().preorderTraversal(node1));
    }

    public List<Integer> preorderTraversal(TreeNode root) {
        if(root!=null){
            values.add(root.val);
            preorderTraversal(root.left);
            preorderTraversal(root.right);
        }
        return values;
    }
}


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;
  }
}