Implement flexible longest sliding window algorithm

Author: oreillyross

algo_templates/sliding_window_flexible_longest.py

@@ -0,0 +1,9 @@
+def sliding_longest_flexible(input):
+  initialize window, ans
+  for right in range(len(input)):
+    append input[right] to window
+    while invalid(window):
+      remove input[left] from window
+      left += 1
+    ans = max(ans, window)
+  return ans