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

public int subarraySum(int[] nums, int k) {
    Map<Integer, Integer> map = new HashMap<>(); // 使用map记录出现同样的和的次数, 对每个i计算累计和sum并判断map内是否有sum-k
    map.put(0,1);
    int sum = 0;
    int res = 0;
    for (int i = 0; i < nums.length; i++)  {
        sum += nums[i];
        res += map.getOrDefault(sum - k, 0);//注意必须是sum-k，
        map.put(sum,map.getOrDefault(sum,0) + 1);
    }
    return res;
}