added Subarray Sum Equals K (medium)

Author: minhyeong-joe

Medium/SubarraySumEqualsK/Driver.java

@@ -0,0 +1,13 @@
+package Medium.SubarraySumEqualsK;
+
+import java.util.Arrays;
+
+public class Driver {
+ public static void main(String[] args) {
+ Solution sol = new Solution();
+ int[] nums = {1,1,1,2,1,2,3,5};
+ int K = 5;
+ System.out.println("Input: nums = " + Arrays.toString(nums) + ", k = " + K);
+ System.out.println("Output: " + sol.subarraySum(nums, K));
+ }
+}

Medium/SubarraySumEqualsK/README.md

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+
+# Subarray Sum Equals K
+[Leetcode Link](https://leetcode.com/problems/subarray-sum-equals-k/)
+
+## Problem:
+
+Given an array of integers and an integer **k**, you need to find the total number of continuous subarrays whose sum equals to **k**.
+
+## Example:
+
+```
+Input:nums = [1,1,1], k = 2
+Output: 2
+```
+
+## Note:
+
+- The length of the array is in range [1, 20,000].
+- The range of numbers in the array is [-1000, 1000] and the range of the integer **k** is [-1e7, 1e7].

Medium/SubarraySumEqualsK/Solution.java

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+package Medium.SubarraySumEqualsK;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class Solution {
+ public int subarraySum(int[] nums, int k) {
+ int numSubarray = 0;
+ int cumSum = 0;
+ // hashmap to keep track of cumulative sums
+ Map<Integer, Integer> sumCount = new HashMap<>();
+ sumCount.put(0,1);
+ for (int num: nums) {
+ cumSum += num;
+ // if cumSum-k appeared in Map, then add count
+ if (sumCount.containsKey(cumSum-k)) {
+ numSubarray += sumCount.get(cumSum-k);
+ }
+ // count cumulative sum
+ if (!sumCount.containsKey(cumSum)) {
+ sumCount.put(cumSum, 1);
+ } else {
+ sumCount.put(cumSum, sumCount.get(cumSum)+1);
+ }
+ }
+ return numSubarray;
+ }
+}

README.md

@@ -67,6 +67,7 @@ Languages used: Java and Python
  - [Asteroid Collision](Medium/AsteroidCollision)
  - [Surrounded Regions](Medium/SurroundedRegions)
  - [Predict the Winner](Medium/PredictWinner)
+ - [Subarray Sum Equals K](Medium/SubarraySumEqualsK)
 - Hard
  - [Maximum Score Words Formed by Letters](Hard/MaximumScoreWords)
  - [Reducing Dishes](Hard/ReducingDishes)