C++ Cheat Sheet Leetcode

5 min read Jul 01, 2024

C++ Cheat Sheet for LeetCode

LeetCode is a popular platform for practicing coding interview questions. While there are many resources available to learn about C++, here's a cheat sheet tailored specifically for solving LeetCode problems in C++.

1. Essential Data Structures and Algorithms

Data Structures

Arrays:
- vector<int> nums = {1, 2, 3};: Create a vector of integers.
- nums.size();: Get the size of the vector.
- nums[i]: Access the element at index i.
- nums.push_back(4);: Add an element to the end.
Linked Lists:
- struct ListNode { int val; ListNode *next; };: Define a linked list node.
- ListNode* head = new ListNode(1);: Create a new linked list node with value 1.
- head->next = new ListNode(2);: Connect the nodes.
Stacks and Queues:
- stack<int> st;: Create a stack.
- st.push(1);: Push an element onto the stack.
- st.top();: Access the top element.
- st.pop();: Remove the top element.
- queue<int> q;: Create a queue.
- q.push(1);: Push an element to the back of the queue.
- q.front();: Access the front element.
- q.pop();: Remove the front element.
Hash Tables (Unordered Maps):
- unordered_map<int, int> hash;: Create an unordered map with integer keys and values.
- hash[1] = 2;: Insert or update a key-value pair.
- hash.count(1);: Check if a key exists.
- hash.erase(1);: Remove a key-value pair.

Algorithms

Sorting:
- sort(nums.begin(), nums.end());: Sort a vector in ascending order.
- sort(nums.begin(), nums.end(), greater<int>());: Sort a vector in descending order.
Binary Search:
- lower_bound(nums.begin(), nums.end(), target);: Find the first element not less than target.
- upper_bound(nums.begin(), nums.end(), target);: Find the first element greater than target.
Depth-First Search (DFS) and Breadth-First Search (BFS):
- Often implemented recursively for DFS and iteratively for BFS using a stack/queue.

2. Useful C++ Techniques

auto: Automatically deduce data type. Example: auto sum = 1 + 2;
const: Declare a constant variable. Example: const int size = 10;
nullptr: Represents a null pointer.
std::swap: Swap the values of two variables.
std::max and std::min: Get the maximum or minimum of two values.
std::accumulate: Calculate the sum of elements in a range.
std::unique: Remove consecutive duplicates in a range.
std::reverse: Reverse the elements in a range.
std::find: Find the first occurrence of an element in a range.

3. Example LeetCode Problem (Two Sum)

#include 
#include 

using namespace std;

vector twoSum(vector& nums, int target) {
    unordered_map seen;
    for (int i = 0; i < nums.size(); ++i) {
        int complement = target - nums[i];
        if (seen.count(complement)) {
            return {seen[complement], i};
        }
        seen[nums[i]] = i;
    }
    return {};
}

4. Common Mistakes

Incorrectly handling edge cases: Remember to handle empty inputs, null pointers, or unexpected data.
Ignoring the constraints: Pay attention to limitations on the input size, range of values, or time complexity.
Misusing data structures: Choose the most appropriate data structure for the problem.
Not testing thoroughly: Test your code with different inputs, including edge cases.

This C++ cheat sheet aims to provide a starting point for tackling LeetCode problems. Remember to practice consistently and familiarize yourself with the specific requirements of each problem. Good luck!