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SubarraySumEqualsK.java
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SubarraySumEqualsK.java
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package Leetcode;
import java.util.HashMap;
import java.util.Map;
/**
* @author kalpak
*
* Given an array of integers nums and an integer k, return the total number of continuous subarrays whose sum equals to k.
*
* Example 1:
*
* Input: nums = [1,1,1], k = 2
* Output: 2
* Example 2:
*
* Input: nums = [1,2,3], k = 3
* Output: 2
*
* Constraints:
*
* 1 <= nums.length <= 2 * 104
* -1000 <= nums[i] <= 1000
* -107 <= k <= 107
*/
public class SubarraySumEqualsK {
public static int subarraySum(int[] nums, int k) {
int[] prefixSum = new int[nums.length + 1];
int runningSum = 0;
Map<Integer, Integer> map = new HashMap<>();
int result = 0;
for (int i = 0; i < nums.length; i++) {
runningSum += nums[i];
prefixSum[i + 1] = runningSum;
}
//now the problem becomes find two items from this sums array so that prefixSum[j]-prefixSum[i] = k (similar to two sum)
for (int i = 0; i < prefixSum.length; i++) {
if(map.containsKey(prefixSum[i]))
result += map.get(prefixSum[i]);
int target = prefixSum[i] + k;
map.put(target, map.getOrDefault(target, 0) + 1);
}
return result;
}
public static int subarraySumEqualsK(int[] nums, int k) {
int runningSum = 0;
int result = 0;
Map<Integer, Integer> map = new HashMap<>();
map.put(0, 1);
for(int i = 0; i < nums.length; i++) {
runningSum += nums[i];
if(map.containsKey(runningSum - k))
result += map.get(runningSum - k);
map.put(runningSum, map.getOrDefault(runningSum, 0) + 1);
}
return result;
}
public static void main(String[] args) {
int[] arr = new int[]{1, 1, 1};
System.out.println(subarraySum(arr, 2));
System.out.println(subarraySumEqualsK(arr, 2));
arr = new int[]{1, 2, 3};
System.out.println(subarraySum(arr, 3));
System.out.println(subarraySumEqualsK(arr, 3));
}
}