中序遍历 递归与非递归

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    public List<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> arr=new ArrayList<Integer>();
        if(root==null) return arr;
       inorder(root,arr);
       return (List)arr;
        
        
    }
     public void inorder(TreeNode root,ArrayList<Integer> arr) {
        if(root!=null)
        {
           inorder(root.left,arr);
           arr.add(root.val);
           inorder(root.right,arr);
        }

        
        
    }
}

2。非递归

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    public List<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> arr=new ArrayList<Integer>();
        if(root==null) return arr;
       Stack<TreeNode> s=new Stack<TreeNode>();
       TreeNode t=root;
       while(!s.isEmpty()||t!=null)
       {
           while(t!=null)
           {
               s.push(t);
               t=t.left;
           }
           
           if(s!=null)
           {
               t=s.pop();
              arr.add(t.val);
              t=t.right;
               
               
           }
           
           
       }
        
        return arr;
    }
    
}