leetcode 94 binary tree inorder traversal

import utils.TreeNode;

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

public class  {
  public List<Integer> inorderTraversalI(TreeNode root) {
    List<Integer> ans = new ArrayList<>();
    travel(root, ans);
    return ans;
  }
  public void travel(TreeNode root, List<Integer> ans) {
    if (root == null) return ;
    travel(root.left, ans);
    ans.add(root.val);
    travel(root.right, ans);
  }

  public List<Integer> inorderTraversalII(TreeNode root) {
    List<Integer> ans = new ArrayList<>();
    Stack<TreeNode> s = new Stack<>();
    TreeNode t = root;
    while (t != null || !s.isEmpty()) {
      while (t!= null) {
        s.push(t);
        t = t.left;
      }
      t = s.pop();
      ans.add(t.val);
      t = t.right;
    }
    return ans;
  }
}