【LeetCode-剑指offer】-- 10.和为K的子数组

10.和为K的子数组

方法一：枚举

class Solution {
    public int subarraySum(int[] nums, int k) {
        int count = 0;
        for (int start = 0; start < nums.length; ++start) {
            int sum = 0;
            for (int end = start; end >= 0; --end) {
                sum += nums[end];
                if (sum == k) {
                    count++;
                }
            }
        }
        return count;
    }
}

方法二：前缀和+哈希

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