LeetCode - Number of Islands

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

Input:
11110
11010
11000
00000

Output: 1

Example 2:

Input:
11000
11000
00100
00011

Output: 3

Java Solution ;

class Solution {
    public int numIslands(char[][] grid) {
        int numOfIslands = 0;
        for(int i =0; i < grid.length;i++){
            for(int j=0; j < grid[i].length;j++){
                if(grid[i][j] == '1')
                numOfIslands += dfs(grid,i,j);
            }
        }
        return numOfIslands;
    }
    public int dfs (char[][] grid , int i , int j){
        if(i < 0 || j < 0 || i >= grid.length || j >= grid[i].length || grid[i][j] == '0'){
            return 0;
        }
        
        grid[i][j] = '0';
        dfs(grid,i+1,j);
        dfs(grid,i-1,j);
        dfs(grid,i,j+1);
        dfs(grid,i,j-1);
        return 1;
    }
}

LeetCode - Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

Java Solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode temp = new ListNode(0);
        int carry = 0;
        ListNode headA =l1 , headB = l2 , curr = temp;
     
           while (headA != null || headB !=null){
                int x = (headA != null) ? headA.val : 0;
                int y = (headB != null) ? headB.val : 0;
                int sum = x + y + carry;
                carry = sum / 10;
                curr.next = new ListNode(sum % 10);     
                curr = curr.next;
               if(headA != null) headA = headA.next;
               if(headB != null) headB = headB.next;
           }
            if(carry > 0)
                curr.next = new ListNode(carry);
            return temp.next;
        
    }
}

Time Complexity : O(max(m,n))

Space Complexity : O(max(m,n))+1

LeetCode - Valid Parentheses

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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

Java Solution :

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        for(char c : s.toCharArray()){
            if(c == '{' || c == '['|| c == '('){
                stack.push(c);
            }else if(c == '}' && !stack.isEmpty() && stack.peek()=='{'){
                stack.pop();
            }else if(c == ']' && !stack.isEmpty() && stack.peek()=='['){
                stack.pop();
            }else if(c == ')' && !stack.isEmpty() && stack.peek()=='('){
                stack.pop();
            }else{
                return false;
            }
        }
        return stack.isEmpty();
    }
}

LeetCode - Most Common Word

Given a paragraph and a list of banned words, return the most frequent word that is not in the list of banned words.  It is guaranteed there is at least one word that isn't banned, and that the answer is unique.

Words in the list of banned words are given in lowercase, and free of punctuation.  Words in the paragraph are not case sensitive.  The answer is in lowercase.

Example:

Input: 
paragraph = "Bob hit a ball, the hit BALL flew far after it was hit."
banned = ["hit"]
Output: "ball"
Explanation: 
"hit" occurs 3 times, but it is a banned word.
"ball" occurs twice (and no other word does), so it is the most frequent non-banned word in the paragraph. 
Note that words in the paragraph are not case sensitive,
that punctuation is ignored (even if adjacent to words, such as "ball,"), 
and that "hit" isn't the answer even though it occurs more because it is banned.

Note:

• 1 <= paragraph.length <= 1000.
• 0 <= banned.length <= 100.
• 1 <= banned[i].length <= 10.
• The answer is unique, and written in lowercase (even if its occurrences in paragraph may have uppercase symbols, and even if it is a proper noun.)
• paragraph only consists of letters, spaces, or the punctuation symbols !?',;.
• There are no hyphens or hyphenated words.
• Words only consist of letters, never apostrophes or other punctuation symbols.

Java Solution:

class Solution {

    public String mostCommonWord(String paragraph, String[] banned) {

        String result =" ";

        Set<String> bannedset = new HashSet<String>();

        for(String word : banned) bannedset.add(word);

        Map<String,Integer> map = new HashMap<String,Integer>();

        String[] parawords = paragraph.replaceAll("[^a-zA-Z]"," ").toLowerCase().split(" ");

        for(String pword : parawords){

            if(!bannedset.contains(pword) && !pword.isEmpty())

                map.put(pword,map.getOrDefault(pword,0)+1);

        }

        

        int max =0;

        for(String word1 : map.keySet()){

            if(map.get(word1)>max){

                max = map.get(word1);

                result = word1;

            }

        }

        return result;

    }

}

LeetCode - Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Java Solution :

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

Time complexity : O(n) Space Complexity O(n)

LeetCode-Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

Iterative Approach :

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode preNode = new ListNode(-1);
        ListNode prev = preNode;
        
        while (l1!=null && l2!=null){
            if(l1.val <= l2.val){
                prev.next = l1;
                l1 = l1.next;
            }else{
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }
        prev.next = l1 == null ? l2 : l1;
        return preNode.next;
    }
}

Time Complexity : O(n+m) Space Complexity O(1)

Recursive Approach:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null){
            return l2;
        }else if(l2 == null){
            return l1;
        }else if(l1.val <l2.val){
                l1.next = mergeTwoLists(l1.next,l2);
            return l1;
            }else{
                l2.next = mergeTwoLists(l1,l2.next);
            return l2;
            }
        }
}

Time Complexity : O(n+m) Space Complexity : O(n+m)

LeetCode - Two Sum Less Than K

Given an array A of integers and integer K, return the maximum S such that there exists i < j with A[i] + A[j] = S and S < K. If no i, j exist satisfying this equation, return -1.

Example 1:

Input: A = [34,23,1,24,75,33,54,8], K = 60
Output: 58
Explanation: 
We can use 34 and 24 to sum 58 which is less than 60.

Example 2:

Input: A = [10,20,30], K = 15
Output: -1
Explanation: 
In this case it's not possible to get a pair sum less that 15.

Note:

1. 1 <= A.length <= 100
2. 1 <= A[i] <= 1000
3. 1 <= K <= 2000

Java Solution ;

class Solution {

    public int twoSumLessThanK(int[] A, int K) {

        Arrays.sort(A);

        int maxSum = -1;

        int i = 0;

        int j = A.length-1;

        while(i < j){

            int sum = A[i]+A[j];

            if(sum<K){

                maxSum = Math.max(maxSum,sum);

                i++;

            }else{

                j--;

            }

        }

     return maxSum;

    }

}