zoukankan      html  css  js  c++  java
  • 298. Binary Tree Longest Consecutive Sequence

    题目:

    Given a binary tree, find the length of the longest consecutive sequence path.

    The path refers to any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The longest consecutive path need to be from parent to child (cannot be the reverse).

    For example,

       1
        
         3
        / 
       2   4
            
             5
    

    Longest consecutive sequence path is 3-4-5, so return 3.

       2
        
         3
        / 
       2    
      / 
     1
    

    Longest consecutive sequence path is 2-3,not3-2-1, so return 2.

    链接: http://leetcode.com/problems/binary-tree-longest-consecutive-sequence/

    题解:

    求二叉树中最长连续序列。 题目又比较长, 不过我们可以确定这个最长连续序列肯定是递增的,比如123,或者345。知道这点以后就可以用DFS遍历了。

    Time Complexity - O(n),  Space Complexity - O(n)。

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    public class Solution {
        private int max = 0;
        public int longestConsecutive(TreeNode root) {
            if(root == null) {
                return max;
            }
            findLongestConsecutive(root, 0, root.val);
            return max;
        }
        
        private void findLongestConsecutive(TreeNode root, int curMax, int target) {
            if(root == null) {
                return;
            }
            if(root.val == target) {
                curMax++;
            } else {
                curMax = 1;
            }
            max = Math.max(max, curMax);
            findLongestConsecutive(root.left, curMax, root.val + 1);
            findLongestConsecutive(root.right, curMax, root.val + 1);
        }
    }

    二刷:

    根据题目的意思,最长连续子序列必须是从root到leaf的方向。 比如 1->2,那么我们就返回长度2, 比如1->3->4->5,我们就返回3->4->5这个子序列的长度3。把树遍历一遍就可以得到结果了。

    方法和一刷一样,构建一个辅助方法,形参是当前要处理的节点root,root父节点的值lastVal,以及当前的长度curLen。也要使用一个global variable maxLen来保存全局最大长度。也有使用stack来iterative遍历的做法。

    Java:

    Time Complexity - O(n),  Space Complexity - O(n)。

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    public class Solution {
        private int maxLen = 0;
        
        public int longestConsecutive(TreeNode root) {
            longestConsecutive(root, 0, 0);
            return maxLen;
        }
        
        private void longestConsecutive(TreeNode root, int lastVal, int curLen) {
            if (root == null) return;
            if (root.val != lastVal + 1) curLen = 1;
            else curLen++;
            maxLen = Math.max(maxLen, curLen);
            longestConsecutive(root.left, root.val, curLen);
            longestConsecutive(root.right, root.val, curLen);
        }
    }

    Reference:

    https://leetcode.com/discuss/68723/simple-recursive-dfs-without-global-variable

    https://leetcode.com/discuss/66486/c-solution-in-4-lines

    https://leetcode.com/discuss/66565/1ms-easy-understand-java-solution-just-traverse-the-tree-once

    https://leetcode.com/discuss/68094/dont-understand-what-is-consecutive-sequence

    https://leetcode.com/discuss/66646/two-simple-iterative-solutions-bfs-and-dfs

    https://leetcode.com/discuss/66548/recursive-solution-bottom-iteration-solution-using-stack

    https://leetcode.com/discuss/66584/easy-java-dfs-is-there-better-time-complexity-solution

  • 相关阅读:
    MongoDB遇到的疑似数据丢失的问题。不要用InsertMany!
    MongoDB-Java的两个基本操作Upsert和insertMany
    MongoDB With Spark遇到的2个错误,不能初始化和sample重复的key
    mongoDB启动前的系统设置,解决部分Warning问题
    使用commons-compress解压GBK格式winzip文件到UTF8,以及错误使用ZipArchiveInputStream读出来数据全是空的解决办法
    mongodb查看操作记录方法以及用户添加删除权限修改密码
    java及spark2.X连接mongodb3.X单机或集群的方法(带认证及不带认证)
    MongoDB3.X单机及shading cluster集群的权限管理(基于3.4.5)
    element ui form表单清空规则
    input type file上传文件之后清空内容。
  • 原文地址:https://www.cnblogs.com/yrbbest/p/5047038.html
Copyright © 2011-2022 走看看