Write a programExercise 4/4Intermediate30 XP

Counting pairs with a sum - fast

Task

Write a function pocet_dvojic(cisla, s) that returns how many pairs of items at different positions of the list cisla add up to s (each pair of positions counts once).

pocet_dvojic([1, 5, 3, 3, 4], 6) returns 2 (1 + 5 and 3 + 3),
pocet_dvojic([2, 2, 2], 4) returns 3.

The list may have 100,000 items - comparing every pair is too slow. A dictionary that remembers how many times each number has already appeared helps.

Examples

Call

print(pocet_dvojic([1, 5, 3, 3, 4], 6))

Expected output

Call
```
print(pocet_dvojic([2, 2, 2], 4))
```
Expected output
```
3
```

After you submit, 6 hidden tests will also check your solution: an empty list, an item does not pair with itself, two equal halves, negative numbers and zero, 100,000 different numbers, 100,000 equal numbers.

You need to know

Python starts the first time you run code.

def pocet_dvojic(cisla, s):
    pass
# Kód pod touto podmienkou sa spustí, keď program spustíš ty. Testy ho preskočia
# a tvoje funkcie zavolajú priamo.
if __name__ == "__main__":
    print(pocet_dvojic([1, 5, 3, 3, 4], 6))

Ctrl+Enter runs the program. Tab indents. Press Esc, then Tab, to leave the editor.

Input (stdin)

Each line = one call to input().

Output

Hints

Try it yourself first. Each hint reveals a little more - and lowers the reward a little.

Signed out, hints are free - but no XP is awarded either.

Solution

The solution unlocks after the last hint.