leet-code/src/main/java/leetcode/editor/cn/BinaryTreeZigzagLevelOrderTraversal.java

//给定一个二叉树，返回其节点值的锯齿形层序遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。 
//
// 例如： 
//给定二叉树 [3,9,20,null,null,15,7], 
//
// 
//    3
//   / \
//  9  20
//    /  \
//   15   7
// 
//
// 返回锯齿形层序遍历如下： 
//
// 
//[
//  [3],
//  [20,9],
//  [15,7]
//]
// 
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package leetcode.editor.cn;

import com.code.leet.entiy.TreeNode;

import java.util.*;

//103:二叉树的锯齿形层序遍历
public class BinaryTreeZigzagLevelOrderTraversal {
    public static void main(String[] args) {
        //测试代码
        Solution solution = new BinaryTreeZigzagLevelOrderTraversal().new Solution();
        List<Integer> list = Arrays.asList(3,9,20,null,null,15,7);
        solution.zigzagLevelOrder(new TreeNode(list));
    }
    //力扣代码
    //leetcode submit region begin(Prohibit modification and deletion)

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     * int val;
     * TreeNode left;
     * TreeNode right;
     * TreeNode() {}
     * TreeNode(int val) { this.val = val; }
     * TreeNode(int val, TreeNode left, TreeNode right) {
     * this.val = val;
     * this.left = left;
     * this.right = right;
     * }
     * }
     */
    class Solution {
        public List<List<Integer>> zigzagLevelOrder(TreeNode root) {List<List<Integer>> ans = new LinkedList<List<Integer>>();
            if (root == null) {
                return ans;
            }

            Queue<TreeNode> nodeQueue = new LinkedList<TreeNode>();
            nodeQueue.offer(root);
            boolean isOrderLeft = true;

            while (!nodeQueue.isEmpty()) {
                Deque<Integer> levelList = new LinkedList<Integer>();
                int size = nodeQueue.size();
                for (int i = 0; i < size; ++i) {
                    TreeNode curNode = nodeQueue.poll();
                    if (isOrderLeft) {
                        levelList.offerLast(curNode.val);
                    } else {
                        levelList.offerFirst(curNode.val);
                    }
                    if (curNode.left != null) {
                        nodeQueue.offer(curNode.left);
                    }
                    if (curNode.right != null) {
                        nodeQueue.offer(curNode.right);
                    }
                }
                ans.add(new LinkedList<Integer>(levelList));
                isOrderLeft = !isOrderLeft;
            }

            return ans;
        }
    }
//leetcode submit region end(Prohibit modification and deletion)

}