树由底层到顶层的层序遍历及之字形层序遍历

题目：牛客网

解题思路：

使用队列进行遍历，栈来递归

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
   public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
	    ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
	    if(root == null){
	    	return res;
	    }
	    Stack<ArrayList<Integer>> stack = new Stack<ArrayList<Integer>>();
	    Queue<TreeNode> queue = new LinkedList<TreeNode>();
	    queue.add(root);
	    while(!queue.isEmpty()){
	    	ArrayList<Integer> arr = new ArrayList<Integer>();
	    	int num = queue.size();
	    	for(int i = 0; i < num; i++){
	    		TreeNode head = queue.poll();
	    		if(head != null){
	    			if(head.left != null)
	    				queue.add(head.left);
	    			if(head.right != null)
	    				queue.add(head.right);
	    		}
	    		arr.add(head.val);
	    	}
	    	stack.add(arr);	
	    }
	    int len = stack.size();
	    for(int i = 0; i < len; i++){
	    	res.add(stack.pop());
	    }
	    return res;
	 }
}

题目：牛客网

解题思路：设置一个flag决定顺序 

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
	    if(root == null){
	    	return res;
	    }
	    Queue<TreeNode> queue = new LinkedList<TreeNode>();
	    queue.add(root);
	    boolean flag = true;
	    while(!queue.isEmpty()){
	    	ArrayList<Integer> arr = new ArrayList<Integer>();
	    	int num = queue.size();
	    	for(int i = 0; i < num; i++){
	    		TreeNode head = queue.poll();
	    		if(head != null){
	    			if(head.left != null)
	    				queue.add(head.left);
	    			if(head.right != null)
	    				queue.add(head.right);
	    		}
	    		if(flag)
	    			arr.add(head.val);
	    		else
	    			arr.add(0,head.val);
	    	}
            flag = !flag;
	    	res.add(arr);	
	    }
	    return res;
    }
}