Binary Tree Zigzag Level Order Traversal 解答

Question

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},

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

Solution

Traditional way is to use two queues to implement level order traversal. Here, we just add a flag to indicate whether it's from left to right or from right to left.

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
         List<List<Integer>> result = new ArrayList<List<Integer>>();
         if (root == null)
             return result;
         // Set a flag to help judge traversal sequence
         // flag = 0, from left to right; flag = 1, from right to left
         int flag = 0;
         List<TreeNode> current = new ArrayList<TreeNode>();
         List<TreeNode> next;
         current.add(root);

         while (current.size() > 0) {
             List<Integer> oneRecord = new ArrayList<Integer>();
             next = new ArrayList<TreeNode>();
             for (TreeNode tmpNode : current) {
                 if (tmpNode.left != null)
                     next.add(tmpNode.left);
                 if (tmpNode.right != null)
                     next.add(tmpNode.right);
                 if (flag == 0)
                     oneRecord.add(tmpNode.val);
                 else
                     oneRecord.add(0, tmpNode.val);
             }
             result.add(oneRecord);
             current = next;
             flag = 1 - flag;
         }
         return result;
     }
 }