#N1475. Final Prices With a Special Discount in a Shop
problem link

#solution

class Solution {
    public int[] finalPrices(int[] prices) {
        Stack<Integer> stack=new Stack<>();
        for(int i=0; i<prices.length; i++){
            while(stack.size()>0&&prices[stack.peek()]>=prices[i]){
                prices[stack.peek()]-=prices[i];
                stack.pop();
            }
            stack.push(i);
        }
        
        return prices;
    }
}

#N1047. Remove All Adjacent Duplicates In String
problem link

#solution

class Solution {
    public String removeDuplicates(String s) {
        StringBuilder res=new StringBuilder();
        Stack<Character> stack=new Stack<>();
        char[] letters=s.toCharArray();
        
        for(int i=letters.length-1; i>=0; i--){
            if(!stack.isEmpty()&&letters[i]==stack.peek())
                stack.pop();
            else
                stack.push(letters[i]);
        }
        
        while(!stack.isEmpty()){
            res.append(stack.pop());
        }
        
        return res.toString();
    }
}

#N1700. Number of Students Unable to Eat Lunch
problem link

#solution

class Solution {
    public int countStudents(int[] students, int[] sandwiches) {
        int[] count=new int[2];
        int i=0;
        
        for(int student:students)
            count[student]++;
        
        for(; i<sandwiches.length && count[sandwiches[i]]!=0; i++)
            count[sandwiches[i]]--;
        
        return sandwiches.length-i;
    }
}

#N933. Number of Recent Calls
problem link

#solution

class RecentCounter {
    Queue<Integer> queue = new LinkedList<>();
    public RecentCounter() {
        
    }
    
    public int ping(int t) {
        while(!queue.isEmpty() && queue.peek()<t-3000) queue.remove();
        
        queue.add(t);
        
        return queue.size();
    }
}

#N1656. Design an Ordered Stream
problem link

#solution

class OrderedStream {
    int index=1;
    Map<Integer, String> map=new HashMap<>();
    public OrderedStream(int n) {
        
    }
    
    public List<String> insert(int key, String value) {
        map.put(key, value);
        List<String> list=new ArrayList<>();
        while(map.containsKey(index)){
            list.add(map.get(index));
            index++;
        }
        
        return list;
    }
}

#N938. Range Sum of BST
problem link

#solution

class Solution {
    int sum=0;
    public int rangeSumBST(TreeNode root, int low, int high) {
        
        if(root == null) {
            return 0;
        }
        if(low<=root.val&&high>=root.val) sum+=root.val;
        
        if(root.val>low){
            rangeSumBST(root.left, low, high);
        }
        if(root.val<=high){
            rangeSumBST(root.right, low, high);
        }
        
        return sum;
    }
}

#N897. Increasing Order Search Tree
problem link

#solution

class Solution {
    TreeNode res = new TreeNode();
    TreeNode curr=res;
    public TreeNode increasingBST(TreeNode root) {
        inOrder(root);
        return res.right;
    }
    
    public void inOrder(TreeNode root){
        if(root == null) return;
        inOrder(root.left);
        curr.right = new TreeNode(root.val);
        curr = curr.right;
        inOrder(root.right);
    }
}

#N700. Search in a Binary Search Tree
problem link

#solution

class Solution {
    public TreeNode searchBST(TreeNode root, int val) {
        if(root == null) return null;
        if(root.val>val){
            root = searchBST(root.left, val);
        }
        else if(root.val<val){
            root = searchBST(root.right, val);
        }
        return root;
    }
}

#N35. Search Insert Position
problem link

#solution

class Solution {
    public int searchInsert(int[] nums, int target) {
        int start=0, end=nums.length-1, mid;
        
        while(start<=end){
            mid=(start+end)/2;
            if(nums[mid]==target) return mid;
            else if(nums[mid]<target) start=mid+1;
            else if(nums[mid]>target) end=mid-1;
        }
        
        return start;
    }
}

#N2057. Smallest Index With Equal Value
problem link

#solution

class Solution {
    public int smallestEqual(int[] nums) {
        for(int i=0; i<nums.length; i++)
            if((i-nums[i])%10==0) return i;
        return -1;
    }
}

#N2078. Two Furthest Houses With Different Colors
problem link

#solution

class Solution {
    public int maxDistance(int[] colors) {
        int x1=0, x2=0;
        for(int i=colors.length-1; i>0; i--)
            if(colors[i]!=colors[0]){
                x1=i;
                break;
            }
        
        for(int i=0; i<colors.length-1; i++)
            if(colors[i]!=colors[colors.length-1]) {
                x2=colors.length-1-i;
                break;
            }
        return Math.max(x1, x2);
    }
}

#statistics
Till this time, 144 in total,
133 easy;
11 medium;
type of problem solutions have been posted

#N2089. Find Target Indices After Sorting Array
problem link

#solution

class Solution {
    public List<Integer> targetIndices(int[] nums, int target) {
        Arrays.sort(nums);
        List<Integer> list = new ArrayList<>();
        
        for(int i=0; i<nums.length; i++)
            if(nums[i]==target) list.add(i);
        
        return list;
    }
}

#N1991. Find the Middle Index in Array
problem link

#solution

class Solution {
    public int findMiddleIndex(int[] nums) {
        int sum = 0, preSum=0, index=-1;
        
        for(int n:nums)
            sum+=n;
        
        for(int i=0; i<nums.length; i++){
            if(sum-nums[i]==preSum){
                return i;
            }
            else{
                preSum+=nums[i];
                sum-=nums[i];
            }
        }
        
        return index;
    }
}

#N724. Find Pivot Index
problem link

#solution

class Solution {
    public int pivotIndex(int[] nums) {
        int sum = 0, preSum=0, index=-1;
        
        for(int n:nums)
            sum+=n;
        
        for(int i=0; i<nums.length; i++){
            if(sum-nums[i]==preSum){
                return i;
            }
            else{
                preSum+=nums[i];
                sum-=nums[i];
            }
        }
        
        return index;
    }
}

#N1822. Sign of the Product of an Array
problem link

#solution

class Solution {
    public int arraySign(int[] nums) {
        long product=1;
        for(int n:nums){
            if(n<0) product = -product;
            if(n==0) return 0;
        }
        
        return signFunc(product);
    }
    
    int signFunc(long product){
        if(product > 0) return 1;
        else if(product < 0) return -1;
        else return 0;
    }
}

#medium

#N807. Max Increase to Keep City Skyline
problem link

#solution

class Solution {
    public int maxIncreaseKeepingSkyline(int[][] grid) {
        int len=grid.length, sum=0;
        int[] rowMax = new int[len];
        int[] colMax = new int[len];
        
        for(int i=0; i<len; i++)
            for(int j=0; j<len; j++)
                rowMax[i] = Math.max(rowMax[i], grid[i][j]);
        
        for(int i=0; i<len; i++)
            for(int j=0; j<len; j++)
                colMax[i] = Math.max(colMax[i], grid[j][i]);
        
        for(int i=0; i<len; i++)
            for(int j=0; j<len; j++)
                sum += Math.min(rowMax[i], colMax[j])-grid[i][j];
        
        return sum;
    }
}

#python
#N1822. Sign of the Product of an Array

#anotherSolution

class Solution:
    def arraySign(self, nums: List[int]) -> int:
        counter = 0
        for i in nums:
            if i == 0:
                return 0
            if i < 0:
                counter +=1
        return 1 if counter % 2 == 0 else -1

#N617. Merge Two Binary Trees
problem link

#solution

class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1 == null && root2 == null) return null;
        else if(root1 == null) return root2;
        else if(root2 == null) return root1;
        
        TreeNode res = new TreeNode(root1.val + root2.val);
        res.left = mergeTrees(root1.left, root2.left);
        res.right = mergeTrees(root1.right, root2.right);
        
        return res;
    }
}

#N2053. Kth Distinct String in an Array
problem link

#solution

class Solution {
    public String kthDistinct(String[] arr, int k) {
        Map<String, Integer> map = new HashMap<>();
        
        for(String s:arr){
            if(map.containsKey(s)) map.put(s, map.get(s)+1);
            else map.put(s, 1);
        }
        
        for(String s:arr){
            if(map.get(s)==1)
                if(k-- == 1)
                    return s;
        }
        
        return "";
    }
}

#recommendation
Advent of Code is a annually virtual event taking place in every December 1th-25th. Every day features new programming exercises, which can be solved in your programming language of choice.
It is full of interesting algorithmic problems, i recommend it)👍🏻

https://adventofcode.com/2021