Top Interview Question by ExpertFunda

Explore the best solutions to the top questions curated by ExpertFunda to help you ace technical interviews.

1. Two Sum

Problem: Given an array of integers nums and an integer target, return the indices of the two numbers such that they add up to target.


public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> numMap = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
        int complement = target - nums[i];
        if (numMap.containsKey(complement)) {
            return new int[]{numMap.get(complement), i};
        }
        numMap.put(nums[i], i);
    }
    throw new IllegalArgumentException("No two sum solution");
}

2. Longest Substring Without Repeating Characters

Problem: Given a string s, find the length of the longest substring without repeating characters.


public int lengthOfLongestSubstring(String s) {
    Map<Character, Integer> charMap = new HashMap<>();
    int left = 0, maxLength = 0;
    for (int right = 0; right < s.length(); right++) {
        char current = s.charAt(right);
        if (charMap.containsKey(current)) {
            left = Math.max(charMap.get(current) + 1, left);
        }
        charMap.put(current, right);
        maxLength = Math.max(maxLength, right - left + 1);
    }
    return maxLength;
}

3. Merge Two Sorted Lists

Problem: Merge two sorted linked lists and return it as a new sorted list.


public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    ListNode dummy = new ListNode(0);
    ListNode current = dummy;
    while (l1 != null && l2 != null) {
        if (l1.val < l2.val) {
            current.next = l1;
            l1 = l1.next;
        } else {
            current.next = l2;
            l2 = l2.next;
        }
        current = current.next;
    }
    current.next = (l1 != null) ? l1 : l2;
    return dummy.next;
}

4. Maximum Subarray

Problem: Find the contiguous subarray within an array, nums, which has the largest sum.


public int maxSubArray(int[] nums) {
    int currentSum = nums[0];
    int maxSum = nums[0];
    for (int i = 1; i < nums.length; i++) {
        currentSum = Math.max(nums[i], currentSum + nums[i]);
        maxSum = Math.max(maxSum, currentSum);
    }
    return maxSum;
}

5. Valid Parentheses

Problem: Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.


public boolean isValid(String s) {
    Stack<Character> stack = new Stack<>();
    Map<Character, Character> mapping = new HashMap<>();
    mapping.put(')', '(');
    mapping.put('}', '{');
    mapping.put(']', '[');

    for (char c : s.toCharArray()) {
        if (mapping.containsKey(c)) {
            char topElement = stack.isEmpty() ? '#' : stack.pop();
            if (topElement != mapping.get(c)) {
                return false;
            }
        } else {
            stack.push(c);
        }
    }
    return stack.isEmpty();
}

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Leetcode Question Solution by ExpertFunda