java实现二叉树的3种遍历方式递归实现

下面xin麒来分享一下二叉树的前中后序遍历递归实现：

前序：

class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;
	public TreeNode() {
	}
	public TreeNode(int val) {
		this.val = val;
	}
	public TreeNode(int val,TreeNode left,TreeNode right) {
		this.val = val;
		this.left = left;
		this.right = right;
	}
}
public class Solution {
    LinkedList<Integer> tree;
    public List<Integer> preorderTraversal(TreeNode root) {
        
        tree = new LinkedList<>();
        pre(root);
        return tree;
    }
    private void pre(TreeNode root){
        if(root == null){return;}
        //xin麒提醒：关键都是以下的三个代码的放置顺序
        tree.add(root.val);
        pre(root.left);
        pre(root.right);
    }
}

中序：

class TreeNode{
     int val;
     TreeNode left;
     TreeNode right;
     public TreeNode(){}
     public TreeNode(int val){this.val = val;}
     public TreeNode(int val,TreeNode left,TreeNode right){
         this.val = val;
         this.left = left;
         this.right = right;
     }
 }
public class Solution {
    LinkedList<Integer> list;
    public List<Integer> inorderTraversal(TreeNode root) {
        list = new LinkedList<>();
        inorder(root);
        return list;
    }
    private void inorder(TreeNode root){
        if (root == null){return;}
        inorder(root.left);
        list.add(root.val);
        inorder(root.right);
    }
}

后序：

 class TreeNode{
     int val;
     TreeNode left;
     TreeNode right;
     public TreeNode(){}
     public TreeNode(int val){this.val = val;}
     public TreeNode(int val,TreeNode left,TreeNode righht){
         this.val = val;
         this.left = left;
         this.right = right;
     }
 }
public class Solution {
    LinkedList<Integer> list;
    public List<Integer> postorderTraversal(TreeNode root) {
        list = new LinkedList<>();
        postoder(root);
        return list;
    }
    private void postoder(TreeNode root){
        if(root == null){return;}
        postoder(root.left);
        postoder(root.right);
        list.add(root.val);
    }
}