I solved this problem using technique which is mentioned in editorial. Then I came up with a different approach: we can iterate through the possible length of a subarray, pick one with the largest sum, update the answer. However, I don't know how to implement finding a subarray with length = 1..n and maximum sum fast enough to pass all tests.
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Same question was asked in this blog 5 years ago: https://codeforces.net/blog/entry/18276