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:

1. The length of the array is in range [1, 20,000].
2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

用hashmap存下之前出現過的sum的個數，遍歷nums時累加num，如果map裏含有sum - k，那麼加上map.get(sum - k)，每次更新sum之後，更新map裏sum出現的次數，map.put(sum, map.getOrDefault(sum, 0) + 1)。代碼如下：

public class Solution {
    public int subarraySum(int[] nums, int k) {
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        int count = 0, sum = 0;
        map.put(0, 1);
        for (int num: nums) {
            sum += num;
            count += map.getOrDefault(sum - k, 0);
            map.put(sum, map.getOrDefault(sum, 0) + 1);
        }
        return count;
    }
}