Array problems in Java.md

Here are some array problems in Java for practice, along with their solutions. These problems cover basic, intermediate, and advanced levels.

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📌 Basic Level

1️⃣ Find the Largest Element in an Array

Problem Statement

Given an integer array nums, find the maximum element in the array.

Example

Input: nums = [3, 7, 2, 9, 5]
Output: 9

Solution

class LargestElement {
    public static int findLargest(int[] nums) {
        int max = nums[0];
        for (int num : nums) {
            if (num > max) {
                max = num;
            }
        }
        return max;
    }

    public static void main(String[] args) {
        int[] nums = {3, 7, 2, 9, 5};
        System.out.println("Largest element: " + findLargest(nums));
    }
}

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📌 Intermediate Level

2️⃣ Move All Zeros to the End

Problem Statement

Given an array nums, move all 0s to the end while maintaining the relative order of non-zero elements.

Example

Input: nums = [0, 1, 0, 3, 12]
Output: [1, 3, 12, 0, 0]

Solution

class MoveZeros {
    public static void moveZerosToEnd(int[] nums) {
        int index = 0;
        for (int num : nums) {
            if (num != 0) {
                nums[index++] = num;
            }
        }
        while (index < nums.length) {
            nums[index++] = 0;
        }
    }

    public static void main(String[] args) {
        int[] nums = {0, 1, 0, 3, 12};
        moveZerosToEnd(nums);
        System.out.println(java.util.Arrays.toString(nums));
    }
}

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3️⃣ Find the Second Largest Element

Problem Statement

Find the second largest element in an array.

Example

Input: nums = [12, 35, 1, 10, 34, 1]
Output: 34

Solution

class SecondLargest {
    public static int findSecondLargest(int[] nums) {
        int first = Integer.MIN_VALUE, second = Integer.MIN_VALUE;
        for (int num : nums) {
            if (num > first) {
                second = first;
                first = num;
            } else if (num > second && num != first) {
                second = num;
            }
        }
        return second == Integer.MIN_VALUE ? -1 : second;
    }

    public static void main(String[] args) {
        int[] nums = {12, 35, 1, 10, 34, 1};
        System.out.println("Second largest: " + findSecondLargest(nums));
    }
}

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📌 Advanced Level

4️⃣ Find the Longest Consecutive Sequence

Problem Statement

Find the length of the longest consecutive sequence in an unsorted array.

Example

Input: nums = [100, 4, 200, 1, 3, 2]
Output: 4  // Sequence: [1, 2, 3, 4]

Solution (Using HashSet)

import java.util.HashSet;

class LongestConsecutive {
    public static int longestConsecutive(int[] nums) {
        HashSet<Integer> set = new HashSet<>();
        for (int num : nums) {
            set.add(num);
        }

        int maxLength = 0;
        for (int num : nums) {
            if (!set.contains(num - 1)) { // Check if it's the start of a sequence
                int currentNum = num;
                int count = 1;

                while (set.contains(currentNum + 1)) {
                    currentNum++;
                    count++;
                }
                maxLength = Math.max(maxLength, count);
            }
        }
        return maxLength;
    }

    public static void main(String[] args) {
        int[] nums = {100, 4, 200, 1, 3, 2};
        System.out.println("Longest Consecutive Sequence: " + longestConsecutive(nums));
    }
}

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5️⃣ Find the Subarray with the Largest Sum (Kadane's Algorithm)

Problem Statement

Find the maximum sum of a subarray using Kadane’s Algorithm.

Example

Input: nums = [-2,1,-3,4,-1,2,1,-5,4]
Output: 6  // Subarray: [4, -1, 2, 1]

Solution

class MaxSubarray {
    public static int maxSubarraySum(int[] nums) {
        int maxSum = nums[0], currentSum = 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;
    }

    public static void main(String[] args) {
        int[] nums = {-2, 1, -3, 4, -1, 2, 1, -5, 4};
        System.out.println("Max Subarray Sum: " + maxSubarraySum(nums));
    }
}

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🏁 Summary of Problems

#	Problem	Difficulty	Time Complexity
1️⃣	Find the Largest Element	Easy	O(n)
2️⃣	Move All Zeros to the End	Medium	O(n)
3️⃣	Find the Second Largest Element	Medium	O(n)
4️⃣	Longest Consecutive Sequence	Hard	O(n)
5️⃣	Max Subarray Sum (Kadane's)	Hard	O(n)

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📚 Next Steps

• Solve variations of these problems.
• Practice with larger constraints.
• Implement these in different programming languages.
• Use sorting and binary search for optimization.

Let me know if you need more problems! 🚀