560. Subarray Sum Equals K (Medium) Google 考简化版，见325

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals tok.

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].

Solution 1: PreSum + HashMap O(n); O(n)

    /**
     * PreSum + HashMap O(n); O(n)
     * if we know SUM[0, i - 1] and SUM[0, j], then we can easily get SUM[i, j].
     * To achieve this, we just need to go through the array, 
     * calculate the current sum and save number of all seen PreSum to a HashMap.
     */
    public int subarraySum(int[] nums, int k) {
        if (nums == null || nums.length == 0) {
            return 0;
        }

        int res = 0;
        int sum = 0;
        Map<Integer, Integer> map = new HashMap<>(); //key: preSum, value: the count of preSum
        map.put(0, 1);
        for (int num : nums) {
            sum += num;
            res += map.getOrDefault(sum - k, 0);
            map.put(sum, map.getOrDefault(sum, 0) + 1);
        }
        return res;
    }