Review the answer

Problem first, then code

Read the problem's rules before the code: what's allowed in the input, and what exactly must come out. Then run the checklist against the code.

AI solutions to famous problems are usually close to right. The bugs hide in the details the problem statement was careful about.

Two Sum

n, target = map(int, input().split())
nums = list(map(int, input().split()))
seen = set(nums)
print("YES" if any(target - x in seen for x in nums) else "NO")

Print YES if numbers at two different positions add up to the target. Would you ship this?

• Yes, it's correct
• No: a number can pair with itself, so 5 with target 10 says YES
• No: it's too slow for 100,000 numbers

Valid brackets

s = input()
stack = []
pairs = {")": "(", "]": "[", "}": "{"}
for c in s:
    if c in "([{":
        stack.append(c)
    elif stack.pop() != pairs[c]:
        print("NO")
        break
else:
    print("YES" if not stack else "NO")

Print YES if every bracket is closed in the right order. Would you ship this?

• Yes, it's correct
• No: it crashes when a closing bracket comes first, like )(
• No: it says YES for (]

Best stretch

n = int(input())
nums = list(map(int, input().split()))
best = cur = nums[0]
for x in nums[1:]:
    cur = max(x, cur + x)
    best = max(best, cur)
print(best)

Print the biggest sum of a run of neighbouring numbers (at least one number). Would you ship this?

• Yes, it's correct
• No: it's wrong when every number is negative
• No: it's wrong for a single number