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TOPIC All advanced algorithm analysis algorithms amortized arrays beginner bfs binary search trees binary-tree binary-trees bst bucket coding-interview complexity analysis computer science cycle data structures data-structures deque dfs dp dsa floyd geometry graph graph-theory greedy greedy algorithms hash map hash set heap in-place intermediate intervals interview prep kadane leetcode linked list matrix mental model monotonic stack performance pointers prefix prefix sum programming fundamentals queue recursion scheduling serialization simulation sliding window sorting stack streaming strings time complexity topo tree trees two pointers worked example
intermediate 6 min · Sep 17, 2026
Learn Subarray sum equals K (prefix sums) explained by tracing how [1, -1, 1] yields 3 valid subarrays using a hash map and running sums.
dsa prefix sum hash map algorithms interview prep
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![How many subarrays of [1, -1, 1] sum to 1 overview diagram](/blogs/subarray-sum-equals-k-explained-counting-1-1-1/diagrams/hero.svg)