Overview

It seems that coding tests are conducted overseas in interviews with engineers, and in many cases, the main thing is to implement specific functions and classes according to the theme.

As a countermeasure, it seems that a site called Let Code will take measures.

A site that trains algorithmic power that can withstand coding tests that are often done in the home.

I think it's better to have the algorithm power of a human being, so I'll solve the problem irregularly and write down the method I thought at that time as a memo.

Leetcode

Basically, I would like to solve the easy acceptance in descending order.

Last time Leet Code Day10 Starting from Zero "1431. Kids With the Greatest Number of Candies"

problem

1315. Sum of Nodes with Even-Valued Grandparent

The difficulty level is Medium.

Given a binary tree, you should return the parent of that parent, the total value of the nodes whose grandparents have an even value. If it does not exist, 0 is returned.

Input: root = [6,7,8,2,7,1,3,9,null,1,4,null,null,null,5] Output: 18 Explanation: The red nodes are the nodes with even-value grandparent while the blue nodes are the even-value grandparents.

Constraints: The number of nodes in the tree is between 1 and 10^4. The value of nodes is between 1 and 100.

An example is like this, with 6 and 8 being the condition of being grandparents and even, and [2,7,1,3] being the grandchildren of 6. The total value of [5], which is the grandson of 8, 18 is returned.

solution

When I saw the problem, I came up with the idea that the current node, the parent node, and the grandparent node are slid and held, and if the grandparent node exists and its value is divided by 2 the remainder is 0. The idea is to add the current node to the total value. The following is an implementation of this idea.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    ans = 0
    def sumEvenGrandparent(self, root: TreeNode) -> int:
        def dfs(cur:TreeNode,par:TreeNode,gra:TreeNode):
            if gra and gra.val % 2 == 0:
                self.ans += cur.val
            if cur.left:
                dfs(cur.left,cur,par)
            if cur.right:
                dfs(cur.right,cur,par)

        dfs(root,None,None)
        return self.ans
# Runtime: 92 ms, faster than 98.47% of Python3 online submissions for Sum of Nodes with Even-Valued Grandparent.
# Memory Usage: 17.3 MB, less than 100.00% of Python3 online submissions for Sum of Nodes with Even-Valued Grandparent.

This time, the way of thinking seemed to be efficient, so I made it very simple. It's fast enough, and I think I got a pretty good answer.

Some people may be curious about the position of ans, but in Python it is not possible to refer to a global variable halfway in a function and redefine it as a local variable halfway, and it is local somewhere in the function. A variable defined as a variable is treated as a local variable from the beginning when it enters the function. So it looks like this.

It was kind of unpleasant, so I looked it up and found out that Python has a nonlocal variable. When defining a function inside a function, it seems to be used to change variables in the outer function from the inner function, and it can be written using it as follows.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def sumEvenGrandparent(self, root: TreeNode) -> int:
        def dfs(cur:TreeNode,par:TreeNode,gra:TreeNode):
            if gra and gra.val % 2 == 0:
                nonlocal ans
                ans += cur.val
            if cur.left:
                dfs(cur.left,cur,par)
            if cur.right:
                dfs(cur.right,cur,par)
        ans = 0
        dfs(root,None,None)
        return ans
# Runtime: 92 ms, faster than 98.47% of Python3 online submissions for Sum of Nodes with Even-Valued Grandparent.
# Memory Usage: 17.3 MB, less than 100.00% of Python3 online submissions for Sum of Nodes with Even-Valued Grandparent.

The speed doesn't change, and this one is cleaner and I personally may like it.

Let Code Day 11 Starting from Zero "1315. Sum of Nodes with Even-Valued Grandparent"

Overview

problem

solution