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;
    }
}