二叉树（前序遍历、中序遍历、后序遍历、层次遍历）

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class PreTraversal{
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] preorderTraversal (TreeNode root) {
        List<Integer> arr = new ArrayList<>();
        preorder(root,arr);

        int[] temp = new int[arr.size()];
        
        for(int i = 0;i<arr.size();i++){
            temp[i] = arr.get(i);
        }

        return temp;
    }

    //前序遍历 值-->左-->右
    public void preorder(TreeNode root,List<Integer> list){
        if(root == null) return;
        list.add(root.val);
        preorder(root.left,list);
        preorder(root.right,list);

    }
}

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] inorderTraversal (TreeNode root) {
        List<Integer> arr = new ArrayList<>();
        inorder(root,arr);
        int[] temp = new int[arr.size()];
        for(int i = 0;i<arr.size();i++){
            temp[i] = arr.get(i);
        }
        // write code here
        return temp;
    }
    //中序遍历 左->值->右
    public void inorder(TreeNode root,List<Integer> list){
        if(root == null) return;
        inorder(root.left,list);
        list.add(root.val);
        inorder(root.right,list);
        
        
        
    }


}

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] postorderTraversal (TreeNode root) {
        // write code here
        List<Integer> arr = new ArrayList<>();
        postoder(arr,root);
        int[] temp = new int[arr.size()];
        for(int i=0;i<arr.size();i++){
            temp[i] = arr.get(i);
        }
        return temp;
    }

    //左->右->取值
    public void postoder(List<Integer> list,TreeNode root){
        if(root == null) return;
        postoder(list,root.left);
        postoder(list,root.right);
        list.add(root.val);
    }
}

    public int[] levelTraversal(TreeNode root){
        if(root == null) return null;
        Queue<TreeNode> q = new LinkedList<>();
        List<Integer> list = new ArrayList<>();
        q.offer(root);
        list.add(root.val);
        while(!q.isEmpty()){
            int n = q.size();
            for(int i = 0; i < n ; i++){
                TreeNode node = q.poll();  //取出节点
                list.add(node.val);
                if(node.left != null){
                    q.offer(node.left);
                }
                if(node.right != null){
                    q.offer(node.right);
                }
            }
        }

        int[] arr = new int[list.size()];
        for(int i=0;i<list.size();i++){
            arr[i] = list.get(i);
        }

        return arr;
    }