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5 LeetCode Problems #1, #121, #20, #53, #70 Explained | DSA Decoded

#programming #python #leetcode #algorithms

🧠 5 LeetCode Problems Explained with Code + Video

Part of the DSA Decoded series — step-by-step Python solutions with video walkthroughs.

Problems covered in this post:

  • 🟢 Two Sum (LeetCode #1) · Easy · Arrays
  • 🟢 Best Time to Buy and Sell Stock (LeetCode #121) · Easy · Arrays
  • 🟢 Valid Parentheses (LeetCode #20) · Easy · Stack
  • 🟡 Maximum Subarray (LeetCode #53) · Medium · Dynamic Programming
  • 🟢 Climbing Stairs (LeetCode #70) · Easy · Dynamic Programming

🟢 Two Sum

Difficulty: Easy  |  Topic: Arrays  |  LeetCode #1

Problem

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. You may assume that each input would have exactly one solution, and you may not use the same element twice.

Solution 

def two_sum(nums, target):
    seen = {}
    for i, n in enumerate(nums):
        comp = target - n
        if comp in seen:
            return [seen[comp], i]
        seen[n] = i
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🔗 Solve on LeetCode  ·  💻 Full solution + tests on GitHub

🟢 Best Time to Buy and Sell Stock

Difficulty: Easy  |  Topic: Arrays  |  LeetCode #121

Problem

You are given an array prices where prices[i] is the price of a given stock on the i-th day. You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock. Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

Solution 

def max_profit(prices):
    min_price = float('inf')
    max_profit = 0
    for price in prices:
        if price < min_price:
            min_price = price
        elif price - min_price > max_profit:
            max_profit = price - min_price
    return max_profit
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🔗 Solve on LeetCode  ·  💻 Full solution + tests on GitHub

🟢 Valid Parentheses

Difficulty: Easy  |  Topic: Stack  |  LeetCode #20

Problem

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if: open brackets must be closed by the same type of brackets, and open brackets must be closed in the correct order.

Solution 

def is_valid(s):
    stack = []
    mapping = {')': '(', '}': '{', ']': '['}
    for char in s:
        if char in mapping:
            top = stack.pop() if stack else '#'
            if mapping[char] != top:
                return False
        else:
            stack.append(char)
    return not stack
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🔗 Solve on LeetCode  ·  💻 Full solution + tests on GitHub

🟡 Maximum Subarray

Difficulty: Medium  |  Topic: Dynamic Programming  |  LeetCode #53

Problem

Given an integer array nums, find the subarray with the largest sum, and return its sum. A subarray is a contiguous non-empty sequence of elements within an array.

Solution 

def max_subarray(nums):
    max_sum = nums[0]
    current_sum = nums[0]
    for num in nums[1:]:
        current_sum = max(num, current_sum + num)
        max_sum = max(max_sum, current_sum)
    return max_sum
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🔗 Solve on LeetCode  ·  💻 Full solution + tests on GitHub

🟢 Climbing Stairs

Difficulty: Easy  |  Topic: Dynamic Programming  |  LeetCode #70

Problem

You are climbing a staircase. It takes n steps to reach the top. Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Solution 

def climb_stairs(n):
    if n <= 2:
        return n
    a, b = 1, 2
    for _ in range(3, n + 1):
        a, b = b, a + b
    return b
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🔗 Solve on LeetCode  ·  💻 Full solution + tests on GitHub

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