题目描述：给定一个二叉树，返回它的中序 遍历。
示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [1,3,2]
解法一：递归（较简单）
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
List<Integer> lis = new ArrayList<>();
public List<Integer> inorderTraversal(TreeNode root) {
if(root == null) return lis; //若节点为空，则返回当前结果数组
inorderTraversal(root.left); //对左子树进行中序遍历
lis.add(root.val);
inorderTraversal(root.right); //对右子树进行中序遍历
return lis;
}
}
解法二：（迭代）--基于栈的中序遍历
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> list = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
TreeNode cur = root;
while (cur != null || !stack.isEmpty()) {
if (cur != null) {
stack.push(cur);//保留当前根节点信息
cur = cur.left;
} else {
cur = stack.pop();
list.add(cur.val);
cur = cur.right;
}
}
return list;
}
}