力扣HOT100-和为K的子数组

思路

含有负数就不能使用滑动窗口！！！

Python版本

class Solution:
    def subarraySum(self, nums: List[int], k: int) -> int:
        n =  len(nums)
        pre = [0] * (n+1)
        cnt = defaultdict(int)
        cnt[0] = 1
        res = 0
        # 构造前缀和
        for i in range(1,n+1):
            pre[i] = pre[i-1] + nums[i-1]
        # 遍历答案
        for j in range(1,n+1):
            res += cnt[pre[j] - k]
            cnt[pre[j]] += 1
        return res

Java版本

class Solution {
    public int subarraySum(int[] nums, int k) {
        int n = nums.length;
        int[] pre = new int[n+1];
        pre[0] = 0;
        for (int i = 1;i<n+1;i++){
            pre[i] = pre[i-1] + nums[i-1];
        }
        int res = 0;
        Map<Integer,Integer> cnt = new HashMap<Integer,Integer>();
        // cnt.put(0,1);
        for (int j = 0;j<n+1;j++){
            res += cnt.getOrDefault(pre[j] - k, 0);
            cnt.put(pre[j],cnt.getOrDefault(pre[j], 0) + 1);
        }
        return res;
    }
}