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LeetCode 103. Binary Tree Zigzag Level Order Traversal

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LeetCode 103. Binary Tree Zigzag Level Order Traversal

103. Binary Tree Zigzag Level Order Traversal

Difficulty: Medium

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,null,null,15,7], 

1
2
3
4
5
  3
 / \
9  20
  /  \
 15   7

return its zigzag level order traversal as: 

1
2
3
4
5
[
  [3],
  [20,9],
  [15,7]
]

Solution

Language: Java

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *       int val;
 *       TreeNode left;
 *       TreeNode right;
 *       TreeNode(int x) { val = x; }
 * }
 */
class Solution {
      public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
            List<List<Integer>> result = new ArrayList<>();
            if (root == null) {
                  return result;
           }
            Queue<TreeNode> q = new LinkedList<>();
            q.offer(root);
            boolean leftToRight = true;
            while (!q.isEmpty()) {
                  int size = q.size();
                  List<Integer> level = new ArrayList<>(size);
                  for (int i = 0; i < size; i++) {
                        TreeNode tmp = q.poll();
                        level.add(tmp.val);
                        if (tmp.left != null) {
                              q.offer(tmp.left);
                       }
                        if (tmp.right != null) {
                              q.offer(tmp.right);
                       }
                 }
                  if (leftToRight) {
                        result.add(level);
                 } else {
                        Collections.reverse(level);
                        result.add(level);
                 }
                  leftToRight = !leftToRight;
           }
            return result;
     }
}