Binary Tree Right Side View

发表于

算法：二叉树广度优先遍历 BFS

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

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题目

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Binary Tree Right Side View

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       <---

Input: [1,2,3,null,5,null,4,null,null,8]
Output: [1, 3, 4, 8]
Explanation:

       1            <---
     /   \
    2     3         <---
     \     \
      5     4       <---
     /
    8               <---

分析

使用层序遍历（level traversal），即广度优先搜索 —— 借助Queue

难点是 标注每一层，来标注 每层的最后一个节点

当前层节点总数：count1，当前访问到节点数 i

下一层几点总数：count2

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Java解法

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

class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        return BFS(root);
    }
    public List<Integer> BFS(TreeNode root) {
        List<Integer> retList = new ArrayList<Integer>();
        if(null == root) return retList;
        int count1=1, count2=0;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while(!queue.isEmpty()) {
            for(int i=0; i<count1; ++i) {
                TreeNode tn = queue.poll();
                if(null != tn) {
                    if (i == count1-1) {
                        retList.add(tn.val);
                    }
                    if(null != tn.left) {
                        queue.add(tn.left);
                        count2++;
                    }
                    if (null != tn.right) {
                        queue.add(tn.right);
                        count2++;
                    }
                }
            }
            count1 = count2;
            count2 = 0;
        }
        return retList;
    }
}

结果

Runtime: 1 ms, faster than 98.22% of Java online submissions for Binary Tree Right Side View.

Memory Usage: 36.3 MB, less than 100.00% of Java online submissions for Binary Tree Right Side View.

1次AC，Niubility

Python解法

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