Let Code Day 32 "437. Path Sum III" starting from scratch

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

Leet Code Table of Contents Starting from Zero

Last time Leet Code Day 31 starting from zero "581. Shortest Unsorted Continuous Subarray"

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

Twitter I'm doing it.

By the way, it lasted for a month. Congratulations.

problem

437. Path Sum III The difficulty level is easy. Easy of Top 100 Liked Questions put this in and there are only 3 questions left.

Each node is given a binary tree that contains an integer value.

Sum the node values to find the number of paths that have a particular value sum.

The route does not have to start or end at the parent or leaf, but it must move downwards (only from the parent node to the child node).

Note that the number of nodes in the tree is 1,000 or less, and the range of values is -1,000,000 to 1,000,000.

root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8


      10
     /  \
    5   -3
   / \    \
  3   2   11
 / \   \
3  -2   1

Return 3. The paths that sum to 8 are:

  1. 5 -> 3
  1. 5 -> 2 -> 1
  2. -3 -> 11

solution

I solved it with dfs using recursion.

# 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 pathSum(self, root: TreeNode, sum: int) -> int:
        def dfs(root,sums,start):
            if not root:
                return 0
            sums -= root.val
            if sums == 0:
                self.ans += 1
            dfs(root.left,sums,False)
            dfs(root.right,sums,False)
            if start:
                dfs(root.left,sum,True)
                dfs(root.right,sum,True)
        dfs(root,sum,True)
        return self.ans
# Runtime: 940 ms, faster than 24.67% of Python3 online submissions for Path Sum III.
# Memory Usage: 15.1 MB, less than 6.82% of Python3 online submissions for Path Sum III.

At first, I thought that I could just call with root and sums, but I couldn't write with that alone (I might be able to write it, but I couldn't think of it now), so when I looked into discuss Almost the same answer, where boolean is used to treat the current node as the starting point. I am using it, and since I was able to implement it very neatly, I will post it as it is.

Even so, people with strong algorithms are amazing ...

You have to work harder.

If there is a good answer, I will add it.

Recommended Posts

Let Code Day 32 "437. Path Sum III" starting from scratch
Let Code Day 39 "494. Target Sum" starting from scratch
Let Code Day 33 "1. Two Sum" starting from scratch
Let Code Day8 starting from scratch "1302. Deepest Leaves Sum"
Let Code Day 65 "560. Subarray Sum Equals K" starting from scratch
Let Code Day4 starting from scratch "938. Range Sum of BST"
Let Code Day 27 "101. Symmetric Tree" starting from scratch
Let Code Day 41 "394. Decode String" starting from scratch
Let Code Day 25 "70. Climbing Stairs" starting from scratch
Let Code Day 34 starting from scratch "118. Pascal's Triangle"
Let Code Day20 starting from scratch "134. Gas Station"
Let Code Day 88 "139. Word Break" starting from scratch
Let Code Day 28 "198. House Robber" starting from scratch
Let Code Day 36 "155. Min Stack" starting from scratch
Let Code Day 17 "169. Majority Element" starting from scratch
Let Code Day 23 "226. Invert Binary Tree" starting from scratch
Let Code Day 22 starting from scratch "141. Linked List Cycle"
Let Code Day 30 starting from scratch "234. Palindrome Linked List"
Let Code Day68 starting from scratch "709. To Lower Case"
Let Code Day 26 starting from scratch "94. Binary Tree Inorder Traversal"
Let Code Day 31 starting from scratch "581. Shortest Unsorted Continuous Subarray"
Let Code Day 38 starting from scratch "208. Implement Trie (Prefix Tree)"
Let Code Day3 starting from scratch "1313. Decompress Run-Length Encoded List"
Let Code Day56 Starting from Zero "5453. Running Sum of 1d Array"
Let Code Day 77 starting from scratch "1502. Can Make Arithmetic Progression From Sequence"
Let Code Day 76 starting from scratch "3. Longest Substring Without Repeating Characters"
Let Code Day58 Starting from Zero "20. Valid Parentheses"
Let Code Day16 Starting from Zero "344. Reverse String"
Let Code Day49 starting from zero "1323. Maximum 69 Number"
Let Code Day89 "62. Unique Paths" Starting from Zero
Let Code Day 55 "22. Generate Parentheses" Starting from Zero
Let Code Day18 starting from zero "53. Maximum Subarray"
Let Code Day 13 "338. Counting Bits" Starting from Zero
Let Code Day 82 "392. Is Subsequence" Starting from Zero
Let Code Day51 Starting from Zero "647. Palindromic Substrings"
Let Code Day 50 "739. Daily Temperatures" Starting from Zero
Let Code Day 15 "283. Move Zeroes" starting from zero
Let Code Day14 starting from zero "136. Single Number"
Let Code Day 9 "701. Insert into a Binary Search Tree" starting from scratch
Let Code Day 80 "703. Kth Largest Element in a Stream" starting from scratch
Let Code Day 66 "438. Find All Anagrams in a String" starting from scratch
Let Code Day 11 Starting from Zero "1315. Sum of Nodes with Even-Valued Grandparent"
Let Code Day 43 "5. Longest Palindromic Substring" Starting from Zero
Let Code Day74 starting from zero "12. Integer to Roman"
Let Code Day 42 "2. Add Two Numbers" Starting from Zero
Let Code Day57 Starting from Zero "35. Search Insert Position"
Let Code Day47 Starting from Zero "14. Longest Common Prefix"
Let Code Day78 Starting from Zero "206. Reverse Linked List"
Let Code Day 90 starting from scratch "101 11. Capacity To Ship Packages Within D Days"
Let Code Day 44 "543. Diameter of Binary Tree" starting from zero
Let Code Day 64 starting from zero "287. Find the Duplicate Number"
Let Code Day 84 starting from zero "142. Linked List Cycle II"
Let Code Day24 Starting from Zero "21. Merge Two Sorted Lists"
Let Code Day12 Starting from Zero "617. Merge Two Binary Trees"
Let Code Day2 Starting from Zero "1108. Defanging an IP Address"
Let Code Day70 Starting from Zero "295. Find Median from Data Stream"
Let Code Day81 "347. Top K Frequent Elements" Starting from Zero
Let Code Day48 Starting from Zero "26. Remove Duplicates from Sorted Array"
Let Code Day87 Starting from Zero "1512. Number of Good Pairs"
Let Code Day67 Starting from Zero "1486. XOR Operation in an Array"
Let Code Day92 Starting from Zero "4. Median of Two Sorted Arrays"