Below is a pattern-focused breakdown of DFS, BFS, and Heap usage tailored for LeetCode-style problems. This is structured for pattern recognition → template → when to use → example problems.

1. DFS Patterns (Trees & Graphs)

Core Idea

• Go deep first, then backtrack
• Uses recursion (stack) or explicit stack
• Good for exploring all possibilities

Pattern 1: Simple Traversal (Tree DFS)

Use When

• Visit all nodes
• Compute something (sum, depth, etc.)

Template

def dfs(node):

if not node:

return

process node

dfs(node.left)

dfs(node.right)

Variations

• Preorder → process before recursion
• Inorder → process between