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[leetcode] problem 103 – binary tree zigzag level order traversal

admin 11月 13, 2020 0

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],

return its zigzag level order traversal as:

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

Code

public class  {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
    
    if (root == null)
        return result;
    
    Deque<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    boolean isOdd = true;

    while (!queue.isEmpty()) {
        int size = queue.size();
        List<Integer> level = new ArrayList<>();

        for (int i = 0; i < size; i++) {
            TreeNode node;

            if (isOdd) {
                node = queue.pollFirst();

                if (node.left != null)
                    queue.offerLast(node.left);
                if (node.right != null)
                    queue.offerLast(node.right);
            }
            else {
                node = queue.pollLast();

                if (node.right != null)
                    queue.offerFirst(node.right);
                if (node.left != null)
                    queue.offerFirst(node.left);
            }

            level.add(node.val);
        }

        result.add(level);
        isOdd = !isOdd;
    }

    return result;
}