Tree BFS — Practice Problems

Ten problems that drill the breadth-first search / level-order pattern from Lesson 12. Open a problem, try it yourself first, then read how to reach the solution. Each solution here was actually run and tested.

How to practise. Memorise the level-by-level skeleton — q = deque([root]), then while q: with an inner for _ in range(len(q)): so you peel off exactly one level per outer step. Once that’s muscle memory, most of these differ only in what you record per level. Notice that Rotting Oranges and Word Ladder are the same BFS on a grid and on a word graph — BFS gives shortest paths when every step costs the same.

Binary-tree problems use this node (LeetCode’s TreeNode); BFS uses deque:

from collections import deque

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right