前缀和 LeetCode 560.和为K的子数组

输入：nums = [1,1,1], k = 2
输出：2

输入：nums = [1,2,3], k = 3
输出：2

public int subarraySum(int[] nums, int k) {
        int count = 0;
        int len = nums.length;
        int[] preSum = new int[len + 1];
        for (int i = 0; i < len; i++) {
            preSum[i + 1] = preSum[i] + nums[i];
        }
        for (int i = 0; i < len; i++) {
            for (int j = i; j < len; j++) {
                if (preSum[j + 1] - preSum[i] == k) {
                    count++;
                }
            }
        }
        return count;
    }

public int subarraySum(int[] nums, int k) {
        int count = 0;
        int preSum = 0;
        Map<Integer, Integer> preSumFreq = new HashMap<>();
        preSumFreq.put(0, 1);
        for (int num : nums) {
//            preSum[i + 1] = preSum[i] + nums[i];
            preSum += num;
//            使用哈希表 省去内层 j 的循环 复杂度降到O(n)
//            for (int j = i; j < len; j++) {
//                  preSum[j + 1] - k == preSum[i]
            count += preSumFreq.getOrDefault(preSum - k, 0);
//            preSum[i + 1] = preSum[i] + nums[i];
            preSumFreq.put(preSum, preSumFreq.getOrDefault(preSum, 0) + 1);
        }
        return count;
    }