Given 2 sorted arrays, pick one element from each to form a tuple. How many tuples are there with sum <= target.
Assume there are no duplicate elements in the arrays.
We can transfer this to a Young's matrix search target problem.
array1 = {1, 3, 6} - n
array2 = {2, 4, 5} - m
target = 7
int[][] matrix: "matrix[i][j] = array1[i] + array2[j]" -> this is a virtual formula
0 1 2
---------
0 | 3 5 6
1 | 5 7 8
2 | 8 10 11
We start from the top right.
for each i (for each row):
find the rightmost j where array1[i] + array2[j] <= target. update j + 1 to the result.
time: O(n + m)
space: O(1)