树的几种遍历方式

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer>list=new ArrayList<>();
        if(root==null) return list;
        list.add(root.val);//根
        List<Integer>left=preorderTraversal(root.left);//左
        List<Integer>right=preorderTraversal(root.right);//右
        list.addAll(left);//添加所有元素到list中
        list.addAll(right);
        return list;
    }
}

import java.util.*;
class Solution {

    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer>list=new ArrayList<>();
        if(root==null) return list;
        Stack<TreeNode> s=new Stack<>();
        s.push(root);
        while(!s.empty()){
            TreeNode node=s.pop();
            list.add(node.val);
            if(node.right!=null) s.push(node.right);
            if(node.left!=null) s.push(node.left);
        }
        return list;
    }
}

 import java.util.*;
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer>list=new ArrayList<>();
        if(root==null) return list;
        List left=inorderTraversal(root.left);//左
        list.addAll(left);
        list.add(root.val);//根
        List right=inorderTraversal(root.right);//右
        list.addAll(right);
        return list;
    }
}

import java.util.*;
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer>list=new ArrayList<>();
        if(root==null) return list;
        Stack<TreeNode>s=new Stack<>();
        TreeNode p=root;
        while(p!=null || !s.empty()){
            if(p!=null){
                s.push(p);
                p=p.left;
            }else{
                p=s.pop();
                list.add(p.val);
                p=p.right;
            }
        }
        return list;
    }
}

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer>list=new ArrayList<>();
        if(root==null) return list;//判空
        List left=postorderTraversal(root.left);
        List right=postorderTraversal(root.right);
        list.addAll(left);//左
        list.addAll(right);//右
        list.add(root.val);//中
        return list;
    }
}