Count flat PnL segments

Question

Given per-trade PnL values, count how many contiguous segments net to exactly zero, stretches where gains and losses perfectly cancel. Return the number of contiguous zero-sum segments. Aim for O(n).

Accepted Answer

A segment (i, j] sums to zero exactly when the running prefix sums at its two ends are equal. So track a running prefix sum and, in a hash map, how many times each prefix value has occurred so far. For each new prefix value, every earlier occurrence of that same value marks the end of a zero-sum segment, so add the stored count before recording the current prefix. Seed the map with prefix 0 occurring once to catch segments starting at index zero. Tracing [1, -1, 2, -2]: prefixes are 1, 0, 2, 0. The first 0 matches the seeded 0 (one segment), and the second 0 matches both earlier 0s (two more)…

Count flat PnL segments

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