LeetCode 560 Subarray Sum Equals K - Walkthrough

Count subarrays summing to $k=3$ in $arr = [1, 2, 1, 2, 1]$. Use prefix sums with a hashmap. As you compute prefix sums, track how many times each sum occurred.

At each position, add $count[prefix - k]$ to the answer. Prefix sums: $0, 1, 3, 4, 6, 7$. At sum $3$: $count[0] = 1$, add $1$. At sum $4$: $count[1] = 1$, add $1$. At sum $6$: $count[3] = 1$, add $1$. At sum $7$: $count[4] = 1$, add $1$. Total: $4$ subarrays. They are: $[1,2]$, $[2,1]$, $[1,2]$, $[2,1]$.