given a list of N integers,
count the number of pairs from this list whose difference is K.
Input: 2 <= N <= 10^5 ; 0 < K < 10^9.
What is the time and space complexity of your approach?
given a m x n matrix of positive integers representing an elevation map,
compute the volume of water it is able to trap after raining.
Example: given [[1, 2, 5, 5], [2, 1, 4, 2], [5, 3, 2, 2]], you should return 1.
Given [[98]], return 0. Given [[95, 2], [1, 17]] return 0.
Given [[3, 5, 5, 3], [5, 1, 3, 6], [3, 8, 8, 3]] return 6.
$ python pairs.py
$ python rainwater.py