2025-11-02
2257. Count Unguarded Cells in the Grid

Topic: Array, Matrix, Simulation
Difficulty: Medium

Problem:
You are given two integers m and n representing a 0-indexed m x n grid. You are also given two 2D integer arrays guards and walls where guards[i] = [row_i, col_i] and walls[j] = [row_j, col_j] represent the positions of the i^th guard and j^th wall respectively.

A guard can see every cell in the four cardinal directions (north, east, south, or west) starting from their position unless obstructed by a wall or another guard. A cell is guarded if there is at least one guard that can see it.

Return the number of unoccupied cells that are not guarded.

Example 1:

Image: https://assets.leetcode.com/uploads/2022/03/10/example1drawio2.png

Input: m = 4, n = 6, guards = [[0,0],[1,1],[2,3]], walls = [[0,1],[2,2],[1,4]]
Output: 7
Explanation: The guarded and unguarded cells are shown in red and green respectively in the above diagram.
There are a total of 7 unguarded cells, so we return 7.

Example 2:

Image: https://assets.leetcode.com/uploads/2022/03/10/example2drawio.png

Input: m = 3, n = 3, guards = [[1,1]], walls = [[0,1],[1,0],[2,1],[1,2]]
Output: 4
Explanation: The unguarded cells are shown in green in the above diagram.
There are a total of 4 unguarded cells, so we return 4.

Constraints:

• 1 <= m, n <= 10^5
• 2 <= m * n <= 10^5
• 1 <= guards.length, walls.length <= 5 * 10^4
• 2 <= guards.length + walls.length <= m * n
• guards[i].length == walls[j].length == 2
• 0 <= row_i, row_j < m
• 0 <= col_i, col_j < n
• All the positions in guards and walls are unique.

2025-11-03
1578. Minimum Time to Make Rope Colorful

Topic: Array, String, Dynamic Programming, Greedy
Difficulty: Medium

Problem:
Alice has n balloons arranged on a rope. You are given a 0-indexed string colors where colors[i] is the color of the i^th balloon.

Alice wants the rope to be colorful. She does not want two consecutive balloons to be of the same color, so she asks Bob for help. Bob can remove some balloons from the rope to make it colorful. You are given a 0-indexed integer array neededTime where neededTime[i] is the time (in seconds) that Bob needs to remove the i^th balloon from the rope.

Return the minimum time Bob needs to make the rope colorful.

Example 1:

Image: https://assets.leetcode.com/uploads/2021/12/13/ballon1.jpg

Input: colors = "abaac", neededTime = [1,2,3,4,5]
Output: 3
Explanation: In the above image, 'a' is blue, 'b' is red, and 'c' is green.
Bob can remove the blue balloon at index 2. This takes 3 seconds.
There are no longer two consecutive balloons of the same color. Total time = 3.

Example 2:

Image: https://assets.leetcode.com/uploads/2021/12/13/balloon2.jpg

Input: colors = "abc", neededTime = [1,2,3]
Output: 0
Explanation: The rope is already colorful. Bob does not need to remove any balloons from the rope.

Example 3:

Image: https://assets.leetcode.com/uploads/2021/12/13/balloon3.jpg

Input: colors = "aabaa", neededTime = [1,2,3,4,1]
Output: 2
Explanation: Bob will remove the balloons at indices 0 and 4. Each balloons takes 1 second to remove.
There are no longer two consecutive balloons of the same color. Total time = 1 + 1 = 2.

Constraints:

• n == colors.length == neededTime.length
• 1 <= n <= 10^5
• 1 <= neededTime[i] <= 10^4
• colors contains only lowercase English letters.

2025-11-04
3318. Find X-Sum of All K-Long Subarrays I

Topic: Array, Hash Table, Sliding Window, Heap (Priority Queue)
Difficulty: Easy

Problem:
You are given an array nums of n integers and two integers k and x.

The x-sum of an array is calculated by the following procedure:

• Count the occurrences of all elements in the array.
• Keep only the occurrences of the top x most frequent elements. If two elements have the same number of occurrences, the element with the bigger value is considered more frequent.
• Calculate the sum of the resulting array.

Note that if an array has less than x distinct elements, its x-sum is the sum of the array.

Return an integer array answer of length n - k + 1 where answer[i] is the x-sum of the subarray nums[i..i + k - 1].

Example 1:

Input: nums = 1,1,2,2,3,4,2,3, k = 6, x = 2

Output: 6,10,12

Explanation:

• For subarray [1, 1, 2, 2, 3, 4], only elements 1 and 2 will be kept in the resulting array. Hence, answer[0] = 1 + 1 + 2 + 2.
• For subarray [1, 2, 2, 3, 4, 2], only elements 2 and 4 will be kept in the resulting array. Hence, answer[1] = 2 + 2 + 2 + 4. Note that 4 is kept in the array since it is bigger than 3 and 1 which occur the same number of times.
• For subarray [2, 2, 3, 4, 2, 3], only elements 2 and 3 are kept in the resulting array. Hence, answer[2] = 2 + 2 + 2 + 3 + 3.

Example 2:

Input: nums = 3,8,7,8,7,5, k = 2, x = 2

Output: 11,15,15,15,12

Explanation:

Since k == x, answer[i] is equal to the sum of the subarray nums[i..i + k - 1].

Constraints:

• 1 <= n == nums.length <= 50
• 1 <= nums[i] <= 50
• 1 <= x <= k <= nums.length

2025-11-05
3321. Find X-Sum of All K-Long Subarrays II

Topic: Array, Hash Table, Sliding Window, Heap (Priority Queue)
Difficulty: Hard

Problem:
You are given an array nums of n integers and two integers k and x.

The x-sum of an array is calculated by the following procedure:

• Count the occurrences of all elements in the array.
• Keep only the occurrences of the top x most frequent elements. If two elements have the same number of occurrences, the element with the bigger value is considered more frequent.
• Calculate the sum of the resulting array.

Note that if an array has less than x distinct elements, its x-sum is the sum of the array.

Return an integer array answer of length n - k + 1 where answer[i] is the x-sum of the subarray nums[i..i + k - 1].

Example 1:

Input: nums = 1,1,2,2,3,4,2,3, k = 6, x = 2

Output: 6,10,12

Explanation:

• For subarray [1, 1, 2, 2, 3, 4], only elements 1 and 2 will be kept in the resulting array. Hence, answer[0] = 1 + 1 + 2 + 2.
• For subarray [1, 2, 2, 3, 4, 2], only elements 2 and 4 will be kept in the resulting array. Hence, answer[1] = 2 + 2 + 2 + 4. Note that 4 is kept in the array since it is bigger than 3 and 1 which occur the same number of times.
• For subarray [2, 2, 3, 4, 2, 3], only elements 2 and 3 are kept in the resulting array. Hence, answer[2] = 2 + 2 + 2 + 3 + 3.

Example 2:

Input: nums = 3,8,7,8,7,5, k = 2, x = 2

Output: 11,15,15,15,12

Explanation:

Since k == x, answer[i] is equal to the sum of the subarray nums[i..i + k - 1].

Constraints:

• nums.length == n
• 1 <= n <= 10^5
• 1 <= nums[i] <= 10^9
• 1 <= x <= k <= nums.length