Top 20 JavaScript related DSA question answers

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 Here’s a list of 20 popular JavaScript-related Data Structures and Algorithms (DSA) questions commonly asked in competitive programming, coding interviews, or during job applications. Along with each question, I’ve provided the solution in JavaScript.

1. Reverse a String

Problem: Reverse a given string.


function reverseString(str) {
    return str.split('').reverse().join('');
}

2. Check for Palindrome

Problem: Check if a string is a palindrome.


function isPalindrome(str) {
    let reversed = str.split('').reverse().join('');
    return str === reversed;
}

3. Fibonacci Sequence

Problem: Find the nth Fibonacci number.


function fibonacci(n) {
    let a = 0, b = 1;
    for (let i = 2; i <= n; i++) {
        [a, b] = [b, a + b];
    }
    return n === 0 ? a : b;
}

4. Find Factorial

Problem: Find the factorial of a number.


function factorial(n) {
    if (n === 0) return 1;
    return n * factorial(n - 1);
}

5. Sum of Natural Numbers

Problem: Find the sum of the first N natural numbers.


function sumOfNumbers(n) {
    return (n * (n + 1)) / 2;
}

6. Binary Search

Problem: Perform binary search on a sorted array.


function binarySearch(arr, target) {
    let left = 0, right = arr.length - 1;
    while (left <= right) {
        const mid = Math.floor((left + right) / 2);
        if (arr[mid] === target) return mid;
        if (arr[mid] < target) left = mid + 1;
        else right = mid - 1;
    }
    return -1;
}

7. Merge Sort

Problem: Sort an array using merge sort.


function mergeSort(arr) {
    if (arr.length <= 1) return arr;

    const mid = Math.floor(arr.length / 2);
    const left = mergeSort(arr.slice(0, mid));
    const right = mergeSort(arr.slice(mid));

    return merge(left, right);
}

function merge(left, right) {
    let result = [], i = 0, j = 0;
    while (i < left.length && j < right.length) {
        if (left[i] < right[j]) result.push(left[i++]);
        else result.push(right[j++]);
    }
    return result.concat(left.slice(i), right.slice(j));
}

8. Quick Sort

Problem: Sort an array using quick sort.


function quickSort(arr) {
    if (arr.length <= 1) return arr;
    const pivot = arr[arr.length - 1];
    const left = [], right = [];
    for (let i = 0; i < arr.length - 1; i++) {
        arr[i] < pivot ? left.push(arr[i]) : right.push(arr[i]);
    }
    return [...quickSort(left), pivot, ...quickSort(right)];
}

9. Merge Two Sorted Arrays

Problem: Merge two sorted arrays.


function mergeArrays(arr1, arr2) {
    let result = [];
    let i = 0, j = 0;
    
    while (i < arr1.length && j < arr2.length) {
        if (arr1[i] < arr2[j]) result.push(arr1[i++]);
        else result.push(arr2[j++]);
    }
    
    return result.concat(arr1.slice(i), arr2.slice(j));
}

10. Two Sum

Problem: Find two numbers in an array that add up to a given target.


function twoSum(nums, target) {
    let map = new Map();
    for (let i = 0; i < nums.length; i++) {
        const complement = target - nums[i];
        if (map.has(complement)) {
            return [map.get(complement), i];
        }
        map.set(nums[i], i);
    }
    return [];
}

11. Reverse Linked List

Problem: Reverse a linked list.


function reverseLinkedList(head) {
    let prev = null, current = head;
    while (current !== null) {
        const nextNode = current.next;
        current.next = prev;
        prev = current;
        current = nextNode;
    }
    return prev;
}

12. Find Middle of Linked List

Problem: Find the middle element of a linked list.


function findMiddle(head) {
    let slow = head, fast = head;
    while (fast !== null && fast.next !== null) {
        slow = slow.next;
        fast = fast.next.next;
    }
    return slow;
}

13. Detect Cycle in Linked List

Problem: Detect if there is a cycle in a linked list.


function hasCycle(head) {
    let slow = head, fast = head;
    while (fast !== null && fast.next !== null) {
        slow = slow.next;
        fast = fast.next.next;
        if (slow === fast) return true;
    }
    return false;
}

14. Find Maximum Subarray (Kadane's Algorithm)

Problem: Find the maximum sum of a contiguous subarray.


function maxSubArray(nums) {
    let maxSoFar = nums[0], maxEndingHere = nums[0];
    for (let i = 1; i < nums.length; i++) {
        maxEndingHere = Math.max(nums[i], maxEndingHere + nums[i]);
        maxSoFar = Math.max(maxSoFar, maxEndingHere);
    }
    return maxSoFar;
}

15. Container With Most Water

Problem: Find the container with the most water.


function maxArea(height) {
    let left = 0, right = height.length - 1;
    let maxArea = 0;
    while (left < right) {
        const area = Math.min(height[left], height[right]) * (right - left);
        maxArea = Math.max(maxArea, area);
        if (height[left] < height[right]) left++;
        else right--;
    }
    return maxArea;
}

16. Find All Anagrams in a String

Problem: Find all the start indices of p's anagrams in s.


function findAnagrams(s, p) {
    let result = [];
    if (s.length < p.length) return result;

    const pCount = Array(26).fill(0);
    const sCount = Array(26).fill(0);

    for (let i = 0; i < p.length; i++) {
        pCount[p.charCodeAt(i) - 97]++;
        sCount[s.charCodeAt(i) - 97]++;
    }

    if (arraysEqual(pCount, sCount)) result.push(0);

    for (let i = p.length; i < s.length; i++) {
        sCount[s.charCodeAt(i) - 97]++;
        sCount[s.charCodeAt(i - p.length) - 97]--;

        if (arraysEqual(pCount, sCount)) {
            result.push(i - p.length + 1);
        }
    }

    return result;
}

function arraysEqual(a, b) {
    for (let i = 0; i < a.length; i++) {
        if (a[i] !== b[i]) return false;
    }
    return true;
}

17. Valid Parentheses

Problem: Check if the parentheses in a string are valid.


function isValid(s) {
    let stack = [];
    for (let char of s) {
        if (char === '(' || char === '[' || char === '{') {
            stack.push(char);
        } else {
            if (stack.length === 0) return false;
            let top = stack.pop();
            if (char === ')' && top !== '(') return false;
            if (char === ']' && top !== '[') return false;
            if (char === '}' && top !== '{') return false;
        }
    }
    return stack.length === 0;
}

18. Find the Intersection of Two Arrays

Problem: Find the intersection of two arrays.


function intersection(nums1, nums2) {
    let set1 = new Set(nums1);
    let result = [];
    for (let num of nums2) {
        if (set1.has(num)) {
            result.push(num);
            set1.delete(num);
        }
    }
    return result;
}

19. Rotate Image (Matrix)

Problem: Rotate an n x n 2D matrix 90 degrees.


function rotate(matrix) {
    let n = matrix.length;
    for (let i = 0; i < Math.floor(n / 2); i++) {
        for (let j = i; j < n - i - 1; j++) {
            let temp = matrix[i][j];
            matrix[i][j] = matrix[n - j - 1][i];
            matrix[n - j - 1][i] = matrix[n - i - 1][n - j - 1];
            matrix[n - i - 1][n - j - 1] = matrix[j][n - i - 1];
            matrix[j][n - i - 1] = temp;
        }
    }
}

20. Longest Substring Without Repeating Characters

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


function lengthOfLongestSubstring(s) {
    let map = new Map();
    let maxLength = 0;
    let start = 0;

    for (let end = 0; end < s.length; end++) {
        if (map.has(s[end])) {
            start = Math.max(map.get(s[end]) + 1, start);
        }
        map.set(s[end], end);
        maxLength = Math.max(maxLength, end - start + 1);
    }

    return maxLength;
}

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