9 Essential LeetCode Patterns to Ace Your Coding Interview

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Preparing for coding interviews requires mastering key problem-solving patterns. Below are nine crucial LeetCode patterns with verified code examples and practical steps to implement them effectively.

1. Two Pointers Pattern

Use Case: Efficiently solve array/string problems with O(n) time complexity.

Example: Removing duplicates from a sorted array.

def removeDuplicates(nums): 
if not nums: 
return 0 
slow = 0 
for fast in range(1, len(nums)): 
if nums[bash] != nums[bash]: 
slow += 1 
nums[bash] = nums[bash] 
return slow + 1

Command to Test:

python3 two_pointers.py

2. Binary Search Pattern

Use Case: Find elements in O(log n) time.

Example: Basic binary search.

def binary_search(arr, target): 
left, right = 0, len(arr) - 1 
while left <= right: 
mid = (left + right) // 2 
if arr[bash] == target: 
return mid 
elif arr[bash] < target: 
left = mid + 1 
else: 
right = mid - 1 
return -1

Command to Test:

python3 binary_search.py

3. Dynamic Programming (DP) Pattern

Use Case: Solve optimization problems by breaking them into subproblems.

Example: Fibonacci sequence with memoization.

def fib(n, memo={}): 
if n in memo: 
return memo[bash] 
if n <= 2: 
return 1 
memo[bash] = fib(n-1, memo) + fib(n-2, memo) 
return memo[bash]

Command to Test:

python3 dp_fibonacci.py

4. Graph Problems for Beginners

Use Case: Traversal (BFS/DFS) and pathfinding.

Example: BFS traversal.

from collections import deque

def bfs(graph, start): 
visited = set() 
queue = deque([bash]) 
while queue: 
node = queue.popleft() 
if node not in visited: 
print(node) 
visited.add(node) 
queue.extend(graph[bash])

Command to Test:

python3 bfs_traversal.py

5. Bit Manipulation Patterns

Use Case: Optimize arithmetic operations.

Example: Check if a number is a power of two.

def is_power_of_two(n): 
return n > 0 and (n & (n - 1)) == 0

Command to Test:

python3 bit_manipulation.py

6. Backtracking Template

Use Case: Generate all possible solutions (e.g., permutations).

Example: Generate permutations.

def permute(nums): 
def backtrack(path, remaining): 
if not remaining: 
res.append(path) 
return 
for i in range(len(remaining)): 
backtrack(path + [remaining[bash]], remaining[:i] + remaining[i+1:]) 
res = [] 
backtrack([], nums) 
return res

Command to Test:

python3 backtracking.py

7. Substring Problems Template

Use Case: Sliding window for substring searches.

Example: Longest substring without repeating characters.

def lengthOfLongestSubstring(s): 
char_set = set() 
left = 0 
max_len = 0 
for right in range(len(s)): 
while s[bash] in char_set: 
char_set.remove(s[bash]) 
left += 1 
char_set.add(s[bash]) 
max_len = max(max_len, right - left + 1) 
return max_len

Command to Test:

python3 sliding_window.py

8. Monotonic Stack/Queue Questions

Use Case: Solve problems like “Next Greater Element.”

Example: Next greater element using a stack.

def nextGreaterElement(nums): 
stack = [] 
res = [-1]  len(nums) 
for i in range(len(nums)): 
while stack and nums[bash] > nums[stack[-1]]: 
res[stack.pop()] = nums[bash] 
stack.append(i) 
return res

Command to Test:

python3 monotonic_stack.py

9. Union-Find Guide

Use Case: Detect cycles or connected components.

Example: Basic Union-Find implementation.

class UnionFind: 
def <strong>init</strong>(self, size): 
self.parent = list(range(size)) 
def find(self, x): 
if self.parent[bash] != x: 
self.parent[bash] = self.find(self.parent[bash]) 
return self.parent[bash] 
def union(self, x, y): 
rootX = self.find(x) 
rootY = self.find(y) 
if rootX != rootY: 
self.parent[bash] = rootX

Command to Test:

python3 union_find.py

What Undercode Say

Mastering these patterns is crucial for coding interviews. Practice implementing them in multiple languages (Python, Java, C++) and optimize for edge cases. Use Linux commands like `time python3 script.py` to measure efficiency.

Expected Output:

  • Two Pointers: `[1, 2, 3]` (after removing duplicates)
  • Binary Search: `4` (index of target)
  • DP Fibonacci: `5` (fib(5))
  • BFS Traversal: `A B C` (graph traversal)
  • Bit Manipulation: `True` (is_power_of_two(16))
  • Backtracking: `[[1,2,3], [1,3,2], …]` (permutations)
  • Sliding Window: `3` (longest substring length)
  • Monotonic Stack: `[5, -1, 4, -1]` (next greater elements)
  • Union-Find: `True` (cycle detection)

Prediction

As coding interviews evolve, these patterns will remain foundational, with increasing emphasis on optimization and real-world system design applications.

References:

Reported By: Fernando Franco – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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