LeetCode.199. 二叉树的右视图

LeetCode.199. 二叉树的右视图

难度：medium

 BFS

很常规的二叉树层序遍历的变型，只需要加入是否时该层最右边元素的判断即可；

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ansList = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        // 如果缺少此判断，会在tmpNode.val处报错
        if (root == null) {
            return ansList;
        }
        queue.offer(root);
        while (!queue.isEmpty()) {
            // List<Integer> levelList = new ArrayList<>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode tmpNode = queue.poll();
                // 判断是否是该层最右边的元素
                if (i == size - 1) {
                    ansList.add(tmpNode.val);
                }
                if (tmpNode.left != null) {
                    queue.offer(tmpNode.left);
                }
                if (tmpNode.right != null) {
                    queue.offer(tmpNode.right);
                }
            }
        }
        return ansList;
    }
}