The code (Ruby) below is a solution for https://leetcode.com/problems/uncrossed-lines/
Note: In Ruby Array#shift is O(1)
The trick is on the lines 14-15:
def max_uncrossed_lines a, b
s = 0
while !a.empty? && a.first == b.first
a.shift
b.shift
s += 1
end
while !a.empty? && a.last == b.last
a.pop
b.pop
s += 1
end
# These two lines
c = a.to_set & b.to_set
a, b = *[a, b].map! { | v | v.filter { c === _1 } }
r0, r1 = *Array.new(2) { [0] * (b.size + 1) }
for x in a
for y, j in b.each_with_index
r1[j + 1] = (x == y) ?
1 + r0[j] :
[r0[j + 1], r1[j]].max
end
r0, r1 = r1, r0
end
s + r0.last
end