# leetcode 560. Subarray Sum Equals K

#### 题目

https://leetcode.com/problems/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

Constraints:

The length of the array is in range [1, 20,000].
The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

#### 源码

``````class Solution {
public:
unordered_map<int,int> m;
int sum[20005];
int res = 0;
int subarraySum(vector<int>& nums, int k) {
sum[0] = 0;
m[0] = 1;
for(int i = 1;i <= nums.size();++i){
int s = sum[i-1]+nums[i-1];
sum[i] = s;
int re = s-k;
if(m.find(re)!=m.end()){
res+=m[re];
}
if(m.find(s)==m.end()){
m[s]=1;
}else{
m[s]++;
}
}
return res;
}
};
``````

# leetcode 974. Subarray Sums Divisible by K 前缀和+同余定理

#### 题目

https://leetcode.com/problems/subarray-sums-divisible-by-k/
Given an array A of integers, return the number of (contiguous, non-empty) subarrays that have a sum divisible by K.

Example 1:

Input: A = [4,5,0,-2,-3,1], K = 5
Output: 7
Explanation: There are 7 subarrays with a sum divisible by K = 5:
[4, 5, 0, -2, -3, 1], [5], [5, 0], [5, 0, -2, -3], [0], [0, -2, -3], [-2, -3]

Note:

1 <= A.length <= 30000
-10000 <= A[i] <= 10000
2 <= K <= 10000

#### 源码

``````class Solution {
public:
int sum[30005];
int res = 0;
map<int,int> m;
int subarraysDivByK(vector<int>& A, int K) {
if(A.size()==0)return res;
sum[0] = 0;
m[0] = 1;
for(int i = 1;i <= A.size();++i){
sum[i] = sum[i-1]+A[i-1];
int yu = sum[i]%K;
if(m.count(yu)==0){
m[yu] = 1;
}else{
m[yu]++;
}
res+=(m[yu]-1);
}
return res;
}
};
``````