Top 10 LeetCode Problems for Aspiring Data Scientists with Detailed Solutions

Introduction

Data Science merges statistical knowledge, domain expertise, and programming proficiency. LeetCode, with its extensive collection of coding challenges, is an excellent resource for aspiring data scientists. Here, we explore the top 10 LeetCode problems, providing detailed explanations and Python solutions.

1. Two Sum

Problem: Given nums, an array of integers, and target, find indices of the two numbers whose sum equals target.

Example: nums = [2, 7, 11, 15], target = 9 Solution: [0, 1] (nums[0] + nums[1] = 2 + 7 = 9)

Python Solution:

def twoSum(nums, target):
    num_to_index = {}
    for index, num in enumerate(nums):
        if target - num in num_to_index:
            return [num_to_index[target - num], index]
        num_to_index[num] = index

2. Best Time to Buy and Sell Stock

Problem: Given prices, an array where prices[i] is the stock price on day i, find the maximum profit you can achieve by buying and selling once.

Example: prices = [7,1,5,3,6,4] Solution: 5 (Buy on day 2 at $1, sell on day 5 at $6)

Python Solution:

def maxProfit(prices):
    min_price, max_profit = float('inf'), 0
    for price in prices:
        min_price = min(min_price, price)
        profit = price - min_price
        max_profit = max(max_profit, profit)
    return max_profit

3. Maximum Subarray

Problem: Find the contiguous subarray within nums (containing at least one number) which has the largest sum.

Example: nums = [-2,1,-3,4,-1,2,1,-5,4] Solution: 6 (subarray [4,-1,2,1])

Python Solution:

def maxSubArray(nums):
    max_sum = curr_sum = nums[0]
    for num in nums[1:]:
        curr_sum = max(num, curr_sum + num)
        max_sum = max(max_sum, curr_sum)
    return max_sum

4. Product of Array Except Self

Problem: Given an array nums, return an array output such that output[i] is equal to the product of all the elements of nums except nums[i].

Example: nums = [1,2,3,4] Solution: [24,12,8,6]

Python Solution:

def productExceptSelf(nums):
    output = [1] * len(nums)
    left, right = 1, 1
    
    for i in range(len(nums)):
        output[i] *= left
        left *= nums[i]

    for i in range(len(nums) - 1, -1, -1):
        output[i] *= right
        right *= nums[i]

    return output

5. Longest Substring Without Repeating Characters

Problem: Find the length of the longest substring without repeating characters in s.

Example: s = "abcabcbb" Solution: 3 (substring "abc")

Python Solution:

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

6. Find All Anagrams in a String

Problem: Given a string s and a non-empty string p, find all the start indices of p's anagrams in s.

Example: s = "cbaebabacd", p = "abc" Solution: [0, 6] (anagrams "cba" and "bac" at indices 0 and 6)

Python Solution:

from collections import Counter

def findAnagrams(s, p):
    res, p_count, s_count = [], Counter(p), Counter(s[:len(p)-1])
    for i in range(len(p) - 1, len(s)):
        s_count[s[i]] += 1
        if s_count == p_count:
            res.append(i - len(p) + 1)
        s_count[s[i - len(p) + 1]] -= 1
        if s_count[s[i - len(p) + 1]] == 0:
            del s_count[s[i - len(p) + 1]]
    return res

7. Top K Frequent Elements

Problem: Given a non-empty array of integers nums, return the k most frequent elements.

Example: nums = [1,1,1,2,2,3], k = 2 Solution: [1,2]

Python Solution:

import heapq
from collections import Counter

def topKFrequent(nums, k):
    count = Counter(nums)
    return heapq.nlargest(k, count.keys(), key=count.get)

8. Kth Largest Element in an Array

Problem: Find the kth largest element in an unsorted array nums.

Example: nums = [3,2,1,5,6,4], k = 2 Solution: 5

Python Solution:

def findKthLargest(nums, k):
    nums.sort()
    return nums[-k]

9. Merge Intervals

Problem: Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals.

Example: intervals = [[1,3],[2,6],[8,10],[15,18]] Solution: [[1,6],[8,10],[15,18]]

Python Solution:

def merge(intervals):
    intervals.sort(key=lambda x: x[0])
    merged = []
    for interval in intervals:
        if not merged or merged[-1][1] < interval[0]:
            merged.append(interval)
        else:
            merged[-1][1] = max(merged[-1][1], interval[1])
    return merged

10. Group Anagrams

Problem: Group anagrams from a list of strings.

Example: strs = ["eat","tea","tan","ate","nat","bat"] Solution: [["bat"],["nat","tan"],["ate","eat","tea"]]

Python Solution:

from collections import defaultdict

def groupAnagrams(strs):
    ans = defaultdict(list)
    for s in strs:
        count = [0] * 26
        for c in s:
            count[ord(c) - ord('a')] += 1
        ans[tuple(count)].append(s)
    return ans.values()

Conclusion

These top 10 LeetCode problems provide a comprehensive challenge set for data scientists to refine their coding skills. Solving these problems will not only improve your problem-solving abilities but also prepare you for data science interviews and real-world scenarios. Happy coding!