解法一

  1. /**
  2.  * Definition for a binary tree node.
  3.  * public class TreeNode {
  4.  *     int val;
  5.  *     TreeNode left;
  6.  *     TreeNode right;
  7.  *     TreeNode(int x) { val = x; }
  8.  * }
  9.  */
  10. class Solution {
  11.     List<Integer> ans;
  12.     public List<Integer> inorderTraversal(TreeNode root) {
  13.         ans = new LinkedList<>();
  14.         inOrder(root);
  15.         return ans;
  16.     }
  18.     private void inOrder(TreeNode root) {
  19.         if (root == null) {
  20.             return;
  21.         }
  22.         inOrder(root.left);
  23.         ans.add(root.val);
  24.         inOrder(root.right);
  25.     }
  26. }