题目：

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]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

解题：

递归算法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public IList<int> InorderTraversal(TreeNode root) {
        List<int> ret = new List<int>();
        if (root == null) return ret;
        ret.AddRange(InorderTraversal(root.left));
        ret.Add(root.val);
        ret.AddRange(InorderTraversal(root.right));
        return ret;
    }
}

迭代算法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public IList<int> InorderTraversal(TreeNode root) {
        List<int> ret = new List<int>();
        Stack<TreeNode> vs = new Stack<TreeNode>();
        if (root == null) return ret;
        TreeNode tmp = root;
        while (tmp != null || vs.Count != 0)
        {
            while (tmp!=null)
            {
                vs.Push(tmp);
                tmp = tmp.left;
            }
            if (vs.Count != 0)
            {
                tmp = vs.Pop();
                ret.Add(tmp.val);
                tmp = tmp.right;
            }
        }
        return ret;
    }
}

来源：

力扣（LeetCode）