前序遍历（递归、非递归）、层序遍历（递归、非递归）

递归：

 1 package 剑指offer.前序遍历;
 2 
 3 import java.util.ArrayList;
 4 
 5 /**
 6  * Created by nick on 2018/10/6.
 7  */
 8 public class Solution {
 9     ArrayList<Integer> res=new ArrayList<Integer>();
10     public void preNode(TreeNode root){
11         if(root==null)
12             return;
13         res.add(root.val);
14         preNode(root.left);
15         preNode(root.right);
16     }
17     
18 }
19 class TreeNode {
20     int val = 0;
21     TreeNode left = null;
22     TreeNode right = null;
23 
24     public TreeNode(int val) {
25         this.val = val;
26 
27     }
28 }

非递归：

 1 package 剑指offer.前序遍历;
 2 
 3 import java.util.ArrayList;
 4 import java.util.Stack;
 5 
 6 /**
 7  * Created by nick on 2018/10/6.
 8  */
 9 public class Solution1 {
10     ArrayList<Integer> res=new ArrayList<Integer>();
11     public ArrayList<Integer> preNode(TreeNode root){
12         Stack<TreeNode> temp=new Stack<TreeNode>();
13         if(root==null)
14             return res;
15         temp.push(root);
16         while (!temp.isEmpty()){
17             TreeNode t=temp.pop();
18             res.add(t.val);
19             if(t.right!=null)
20                 temp.push(root.right);
21             if(t.left!=null)
22                 temp.push(root.left);
23 
24         }
25         return res;
26     }
27 
28     public static void main(String[] args) {
29         TreeNode root=new TreeNode(1);
30         root.left=new TreeNode(2);
31         root.right=new TreeNode(3);
32 
33         System.out.println(new Solution1().preNode(root));
34     }
35 }

层序遍历：

    /*
     * 层序遍历
     * 递归
     */
    public void levelOrder(BinaryNode<AnyType> Node) {
        if (Node == null) {
            return;
        }

        int depth = depth(Node);

        for (int i = 1; i <= depth; i++) {
            levelOrder(Node, i);
        }
    }

    private void levelOrder(BinaryNode<AnyType> Node, int level) {
        if (Node == null || level < 1) {
            return;
        }

        if (level == 1) {
            System.out.print(Node.element + "  ");
            return;
        }

        // 左子树
        levelOrder(Node.left, level - 1);//直到第level层，再进行打印

        // 右子树
        levelOrder(Node.right, level - 1);
    }

    public int depth(BinaryNode<AnyType> Node) {
        if (Node == null) {
            return 0;
        }

        int l = depth(Node.left);
        int r = depth(Node.right);
        if (l > r) {
            return l + 1;
        } else {
            return r + 1;
        }
    }

非递归算法：

    //使用队列
    public static void levelTraversal2(TreeNode root)
    {
        ArrayDeque<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        while(!queue.isEmpty())
        {
            TreeNode temp = queue.poll();
            System.out.print(temp.val);
            if(temp.left != null)
            {
                queue.offer(temp.left);
            }
            if(temp.right != null)
            {
                queue.offer(temp.right);
            }
        }
    }
}