More algorithms today. Pretty efficient ones at that. All above 90% ranked on the first submission. I’m beginning to find a lot of patterns that can be applied to the different problems. Most of the Easy binary tree questions are guilty of this.
I’m really happy that I finally solved Flipping an Image — I had been in the thicket of a messy solution for a couple days but I was able to Pythonize it (read: refactor it)! I got hung up on solving it optimally before peaking at any resources.
Problem: Flip a binary matrix and then invert it.
The trick is doing both steps at once. In other words, you don’t want two for loops.
class Solution(object): def flipAndInvertImage(self, A): """ :type A: List[List[int]] :rtype: List[List[int]] """ for x in range(len(A)): A[x] = A[x][::-1] for y in range(len(A[x])): A[x][y] ^= 1 return A
Outrageously straightforwards when it’s reduced down to four lines.
Problem: Do these two trees have similar leaves — read left-to-right.
We’re going to the edge of these trees and comparing the concatenated values found therein, or, there-out-there.
# Definition for a binary tree node. # class TreeNode(object): # def __init__(self, x): # self.val = x # self.left = None # self.right = None class Solution(object): def leafSimilar(self, root1, root2): """ :type root1: TreeNode :type root2: TreeNode :rtype: bool """ self.root1_store =  self.root2_store =  def bfs_recurse(node, store): if not node.left and not node.right: store.append(node.val) else: if node.left: bfs_recurse(node.left, store) if node.right: bfs_recurse(node.right, store) if root1: bfs_recurse(root1, self.root1_store) if root2: bfs_recurse(root2, self.root2_store) return self.root1_store == self.root2_store
First try, pre-optimization:
Runtime: 32 ms, faster than 99.02% of Python online submissions..
Problem: What is the deepest level of this n-ary tree?
For this, we’ll need a full tree traversal and some way of keeping track of the maximum depth so far. Lately, I’ve been reaching for instance variables in situations like this.
""" # Definition for a Node. class Node(object): def __init__(self, val, children): self.val = val self.children = children """ class Solution(object): def maxDepth(self, root): """ :type root: Node :rtype: int """ self.depth = 0 def dfs_recurse(node, depth): depth += 1 for c in node.children: dfs_recurse(c, depth) self.depth = max(depth, self.depth) if root: dfs_recurse(root, 0) return self.depth
I’m going to check out some other solutions that don’t use pseudo-globals.