https://leetcode.com/problems/binary-tree-right-side-view/

Thoughts

We need a BFS for the tree,
traverse each level add the node to a Deque, return the head of deque

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/**
 * 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<Integer> rightSideView(TreeNode root) {
        List<Integer> lst = new ArrayList<>();
        if (root == null) {
            return lst;
        }
        Queue<TreeNode> dq = new LinkedList<>();
        dq.offer(root);
        TreeNode lastNode = null;
        while (dq.size() != 0) {
            int levelSize = dq.size();
            for (int i = 0; i < levelSize; i++) {
                lastNode = dq.poll();
                if (lastNode.left != null) {
                    dq.offer(lastNode.left );
                }
                if (lastNode.right!= null) {
                    dq.offer(lastNode.right);
                }
            }
            lst.add(lastNode.val);
        }
        return lst;
    }
}