560. Subarray Sum Equals K

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].

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注意一些特殊情况,（会对代码的顺序有一点影响）
sum == k
k == 0

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