Binary Tree Zigzag Level Order Traversal

最新推荐文章于 2024-01-19 11:15:23 发布

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本文介绍了一种算法，用于实现二叉树节点值的锯齿形层序遍历。从左到右再从右到左交替进行，并提供了一个具体的示例来展示如何实现这一遍历方式。

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
   public static List<List<Integer>> zigzagLevelOrder(TreeNode root) {
		
        LinkedList<TreeNode> stack=new LinkedList<TreeNode>();  
        LinkedList<TreeNode> curr=new LinkedList<TreeNode>();  
        List<List<Integer>> res=new ArrayList< List<Integer> >();  
        if(root==null) return res;  
        stack.add(root);  
        int flag=0;  
        while(!stack.isEmpty()){  
            curr=stack;  
            stack=new LinkedList<TreeNode>();  
            List<Integer> a=new LinkedList<Integer>();  
            while(curr.size()>0){
                
                if(flag==1){
                    TreeNode tmpNode=null;
                	tmpNode=curr.peekLast();
                	if(tmpNode.right!=null) stack.addFirst(tmpNode.right); 
                	if(tmpNode.left!=null) stack.addFirst(tmpNode.left);  
                    a.add(tmpNode.val);
                    //System.out.print("flag="+flag+" val="+tmpNode.val+" ");
                    curr.pollLast();
                }
                else if(flag==0){
                    TreeNode tmpNode=null;
                	tmpNode=curr.peekFirst();
                	if(tmpNode.left!=null) stack.addLast(tmpNode.left);  
                    if(tmpNode.right!=null) stack.addLast(tmpNode.right);  
                    //System.out.print("flag="+flag+" val="+tmpNode.val+" ");
                    a.add(tmpNode.val);
                	curr.pollFirst(); 
                }
                
            } 
            //System.out.println();
            res.add(a);
            flag=1-flag;
        }  
        return res;   
    }
}