前缀和-prefixsum

560. Subarray Sum Equals K Medium

Given an array of integers nums and an integer k, return the total number of continuous subarrays whose sum equals to k.

Example 1:

Input: nums = [1,1,1], k = 2
Output: 2

Example 2:

Input: nums = [1,2,3], k = 3
Output: 2 

Constraints:

• 1 <= nums.length <= 2 * 104
• -1000 <= nums[i] <= 1000
• -107 <= k <= 107、

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

 

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

 

560. Subarray Sum Equals K Medium

Given an array of integers nums and an integer k, return the total number of continuous subarrays whose sum equals to k.

Example 1:

Input: nums = [1,1,1], k = 2
Output: 2

Example 2:

Input: nums = [1,2,3], k = 3
Output: 2

 Constraints:

• 1 <= nums.length <= 2 * 104
• -1000 <= nums[i] <= 1000
• -107 <= k <= 107

class Solution {
    public int subarraysDivByK(int[] nums, int k) {
        Map<Integer,Integer> map = new HashMap();
        int count = 0;
        int sum=0;
        map.put(0,1);
        for(int i=0;i<nums.length;i++){
            sum=(sum+nums[i]%k+k)%k;//这里加k是为了防止负数
            count += map.getOrDefault(sum,0);
            map.put(sum,map.getOrDefault(sum,0)+1);
        }
        return count;
    }
}

 

标签：前缀,nums,int,prefixsum,new,Output,Input,Example
来源： https://www.cnblogs.com/cynrjy/p/15757068.html