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:
- 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].
解法一:利用sum之间的差暴力搜索
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; } }
解法二:利用map建立sum和出现次数cnt的映射关系
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
作者:Pickle
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