# LeetCode - 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


Note:

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

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

class Solution {
public int subarraySum(int[] nums, int k) {
if (nums == null || nums.length == 0)
return 0;
Map<Integer, Integer> map = new HashMap<>();
map.put(0, 1);
int sum = 0, cnt = 0;
for (int num : nums) {
sum += num;
cnt += map.getOrDefault(sum-k, 0);
map.put(sum, map.getOrDefault(sum, 0)+1);
}
return cnt;
}
}

第二种解法耗时更少

posted @ 2018-10-15 16:32  Pickle  阅读(220)  评论(0编辑  收藏  举报