Given an array points containing the coordinates of points on a 2D plane, sorted by the x-values, where points[i] = [xi, yi] such that xi < xj for all 1 <= i < j <= points.length. You are also given an integer k.

Find the maximum value of the equation yi + yj + |xi - xj| where |xi - xj| <= k and 1 <= i < j <= points.length. It is guaranteed that there exists at least one pair of points that satisfy the constraint |xi - xj| <= k.

Example 1:

Input: points = [[1,3],[2,0],[5,10],[6,-10]], k = 1
Output: 4
Explanation: The first two points satisfy the condition |xi - xj| <= 1 and if we calculate the equation we get 3 + 0 + |1 - 2| = 4. Third and fourth points also satisfy the condition and give a value of 10 + -10 + |5 - 6| = 1.
No other pairs satisfy the condition, so we return the max of 4 and 1.

Example 2:

Input: points = [[0,0],[3,0],[9,2]], k = 3
Output: 3
Explanation: Only the first two points have an absolute difference of 3 or less in the x-values, and give the value of 0 + 0 + |0 - 3| = 3.


Constraints:

• 2 <= points.length <= 10^5
• points[i].length == 2
• -10^8 <= points[i][0], points[i][1] <= 10^8
• 0 <= k <= 2 * 10^8
• points[i][0] < points[j][0] for all 1 <= i < j <= points.length
• xi form a strictly increasing sequence.

Observation

Since xj > xi, so |xi – xj| + yi + yj => xj + yj + (yi – xi)
We want to have yi – xi as large as possible while need to make sure xj – xi <= k.

Solution 1: Priority Queue / Heap

Put all the points processed so far onto the heap as (y-x, x) sorted by y-x in descending order.
Each new point (x_j, y_j), find the largest y-x such that x_j – x <= k.

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

Solution 2: Monotonic Queue

Maintain a monotonic queue:
1. The queue is sorted by y – x in descending order.
2. Pop then front element when xj – x_front > k, they can’t be used anymore.
3. Record the max of {xj + yj + (y_front – x_front)}
4. Pop the back element when yj – xj > y_back – x_back, they are smaller and lefter. Won’t be useful anymore.
5. Finally, push the j-th element onto the queue.

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

python3

If you like my articles / videos, donations are welcome.

Buy anything from Amazon to support our website

Paypal
Venmo
huahualeetcode

Be First to Comment

Mission News Theme by Compete Themes.