binary-tree-zigzag-level-order-traversal

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
         ArrayList<ArrayList<Integer>> res=new  ArrayList<ArrayList<Integer>>();
        if(root==null){
            return res;
        }
        Stack<TreeNode> stack1=new Stack<TreeNode>();
        Stack<TreeNode> stack2=new Stack<TreeNode>();
        stack1.push(root);
        while(!stack1.isEmpty()||!stack2.isEmpty()){
            ArrayList<Integer> temp=new ArrayList<Integer>();
            while(!stack1.isEmpty()){
                TreeNode curr=stack1.pop();
                temp.add(curr.val);
                if(curr.left!=null){
                    stack2.push(curr.left);
                }
                if(curr.right!=null){
                    stack2.push(curr.right);
                }
            }
            if(!temp.isEmpty()){
                  res.add(temp);
            }
              
             temp=new ArrayList<Integer>();
                while(!stack2.isEmpty()){
                   
                      TreeNode curr=stack2.pop();
                    temp.add(curr.val);
               
                if(curr.right!=null){
                    stack1.push(curr.right);
                }
                     if(curr.left!=null){
                    stack1.push(curr.left);
                }
                }
           if(!temp.isEmpty()){
                  res.add(temp);
            }
        }
        return res;
    }
}

使用两个栈，第一层正序，第二层倒叙，遍历第一层时候是正序遍历，第二层是倒序遍历，所以第一层遍历时先把左子树压入，再压入右子树，这样遍历第二层的时候是先把右子树弹出，由于第三层是正序，于是要先把右子树压入，再把左子树压入。