560. 和为K的子数组

题目链接：

https://leetcode-cn.com/problems/subarray-sum-equals-k/

解题思路：

暴力

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

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