package com.stevesun.solutions;

import java.util.HashMap;
import java.util.Map;

/**
 * 560. Subarray Sum Equals K
 *
 * 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 1:
 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].
 */
public class _560 {

    public int subarraySum(int[] nums, int k) {
        Map<Integer, Integer> preSum = new HashMap();
        int sum = 0;
        int result = 0;
        preSum.put(0, 1);
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            if (preSum.containsKey(sum - k)) {
                result += preSum.get(sum-k);
            }
            preSum.put(sum, preSum.getOrDefault(sum, 0) + 1);
        }
        return result;
    }

}

package com.stevesun.solutions;

import java.util.HashMap;
import java.util.Map;

/**
 * 560. Subarray Sum Equals K
 *
 * 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 1:
 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].
 */
public class _560 {

    //credit: https://discuss.leetcode.com/topic/87850/java-solution-presum-hashmap
    public int subarraySum(int[] nums, int k) {
        Map<Integer, Integer> preSum = new HashMap();
        int sum = 0;
        int result = 0;
        preSum.put(0, 1);
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            if (preSum.containsKey(sum - k)) {
                result += preSum.get(sum-k);
            }
            preSum.put(sum, preSum.getOrDefault(sum, 0) + 1);
        }
        return result;
    }

}

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