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leetCode-199:Binary Tree Right Side View

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问题描述

给定一个二叉树，要求按照从上到下的顺序输出其右视图的节点值。题目链接：**点我**

样例输入输出

输入： [1,2,3,null,5,null,4]

输出：[1, 3, 4]

说明：

1 <—-

/ \

2 3 <—-

\ \

5 4 <—-

输入：[1,null,3]

输出：[1,3]

问题解法

此题就是求出树的每一层中最后节点。使用广搜的做法，从上到下依次输出每层的最后一个节点值即可。代码如下

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/**
 * 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> result = new LinkedList<>();
        if (root == null)
        {
            return result;
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int nextCount = 1;
        while (!queue.isEmpty())
        {
            TreeNode current = null;
            int count = 0;
            for (int i = nextCount; i > 0; i--)
            {
                current = queue.poll();
                if (current.left != null)
                {
                    queue.add(current.left);
                    count++;
                }

                if (current.right != null)
                {
                    queue.add(current.right);
                    count++;
                }
            }
            
            nextCount = count;
            if (current != null)
            {
                result.add(current.val);
            }
        }
        
        return result;
    }
}