二叉树先中后序遍历（递归非递归）

Posted 2022-10-31 雾晴

先序遍历

先序遍历也叫先根遍历，采用先访问根节点，再访问左节点，最后访问右节点

递归实现

/**
 * Definition for a binary tree node.
 * public class TreeNode 
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() 
 *     TreeNode(int val)  this.val = val; 
 *     TreeNode(int val, TreeNode left, TreeNode right) 
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     
 * 
 */
class Solution 
    public List<Integer> preorderTraversal(TreeNode root) 
        List<Integer>list=new LinkedList<Integer>();
        bl(root,list);
        return list;
    
    public void bl(TreeNode root,List<Integer> list)
        if(root==null)
            return;
        
        list.add(root.val);
        bl(root.left,list);
        bl(root.right,list);
    

非递归实现

非递归实现借助了栈

/**
 * Definition for a binary tree node.
 * public class TreeNode 
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() 
 *     TreeNode(int val)  this.val = val; 
 *     TreeNode(int val, TreeNode left, TreeNode right) 
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     
 * 
 */
class Solution 
    public List<Integer> preorderTraversal(TreeNode root) 
        return xhbl(root);
    
    public List<Integer> xhbl(TreeNode root)
        List<Integer> list=new ArrayList<>();
        Stack<TreeNode> stack=new Stack<>();

        while(root!=null || !stack.isEmpty())
            
            while (root!=null)
                list.add(root.val);
                stack.push(root.right);
                root=root.left;
            

            root=stack.pop();
            //root=root.right;
        
        return list;
    

中序遍历

中序遍历的顺序为 左节点，根节点、右节点

递归实现

/**
 * Definition for a binary tree node.
 * public class TreeNode 
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() 
 *     TreeNode(int val)  this.val = val; 
 *     TreeNode(int val, TreeNode left, TreeNode right) 
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     
 * 
 */
class Solution 
    public List<Integer> inorderTraversal(TreeNode root) 
        List<Integer> list=new LinkedList<Integer>();
        accTre(root,list);
        return list;
    

    public void accTre(TreeNode res,List<Integer> list)
        if(res==null)
            return ;
        
        accTre(res.left,list);
        list.add(res.val);
        accTre(res.right,list);
    

非递归实现

当然非递归实现也是采用了栈

/**
 * Definition for a binary tree node.
 * public class TreeNode 
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() 
 *     TreeNode(int val)  this.val = val; 
 *     TreeNode(int val, TreeNode left, TreeNode right) 
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     
 * 
 */
class Solution 
    public List<Integer> inorderTraversal(TreeNode root) 
        return xhbl(root);
    

   public List<Integer> xhbl(TreeNode root)
        List<Integer> list=new ArrayList<>();
        Stack<TreeNode> stack=new Stack<>();

        while(root!=null || !stack.isEmpty())
            while (root!=null)
                stack.push(root);
                root=root.left;
            
            
            root=stack.pop();
            list.add(root.val);
            root=root.right;
        
        return list;
    

后序遍历

后续遍历的顺序是左节点、右节点、根节点

递归实现

/**
 * Definition for a binary tree node.
 * public class TreeNode 
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() 
 *     TreeNode(int val)  this.val = val; 
 *     TreeNode(int val, TreeNode left, TreeNode right) 
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     
 * 
 */
class Solution 
    public List<Integer> postorderTraversal(TreeNode root) 
        List<Integer> list=new LinkedList<Integer>();
        hxbl(root,list);
        return list;
    

    public void hxbl(TreeNode root,List<Integer> list)
        if(root==null)
            return;
        

        hxbl(root.left,list);
        hxbl(root.right,list);
        list.add(root.val);

    

非递归实现

非递归实现采用栈和临时变量，
用临时变量标识当前节点的右子节点是否被访问过，从而确定是否将当前节点输出

/**
 * Definition for a binary tree node.
 * public class TreeNode 
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() 
 *     TreeNode(int val)  this.val = val; 
 *     TreeNode(int val, TreeNode left, TreeNode right) 
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     
 * 
 */
class Solution 
    public List<Integer> postorderTraversal(TreeNode root) 
        
        return xhbl(root);
    

    public List<Integer> xhbl(TreeNode root)

        List<Integer> list=new ArrayList<>();
        Stack<TreeNode> stack=new Stack<>();
        TreeNode press = null;

        while(root!=null || !stack.isEmpty())
            while (root!=null)
                stack.push(root);
                root=root.left;
            

            root=stack.pop();
            if(root.right==null || root.right==press)
                list.add(root.val);
                press=root;
                root=null;
            else 
                stack.push(root);
                root=root.right;
            
        
        return list;