第71题 Binary Tree Zigzag Level Order Traversal

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#队列 #queue #算法 #二叉树

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Binary Tree Zigzag Level Order Traversal

Description
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).

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A Tree
     * @return: A list of lists of integer include the zigzag level order traversal of its nodes' values.
     */
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        // write your code here
         Queue<TreeNode> queue = new LinkedList<TreeNode>();
        //List<Integer> level = new ArrayList<Integer>();
        List<List<Integer>> results = new ArrayList<>();
        if(root == null){
           return results ;
        }
        queue.offer(root);
        int count = 0 ;
        while(!queue.isEmpty()){
           List<Integer> level = new ArrayList<Integer>();
           int size = queue.size();
           for(int i = 0 ; i < size ; i++){
             
              TreeNode node = queue.poll() ;
              level.add(node.val);
              if(node.left != null){
                 queue.offer(node.left);
              }
              if(node.right != null){
                 queue.offer(node.right);
              }
           }
           count++ ;
           if(count % 2 == 0){
              Collections.reverse(level) ;
           }
           results.add(level);
        }
        return results;
    }
}