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[leetcode] problem 199 – binary tree right side view

admin 11月 13, 2020 0

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

Example

Input: [1,2,3,null,5,null,4]

Output: [1, 3, 4]

Explanation:

   1            <---
 /   
2     3         <---
      
  5     4       <---

Code

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

public List<Integer> rightSideView(TreeNode root) {
    List<Integer> result = new ArrayList<>();

    if (root == null)
        return result;

    LinkedList<TreeNode> queue = new LinkedList<>();
    queue.offer(root);

    while (!queue.isEmpty()) {
        int size = queue.size();
        result.add(queue.peekLast().val);

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

            if (node.left != null)
                queue.offer(node.left);

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

    return result;
}