leetcode题解-560. Subarray Sum Equals K
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题目:
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 = 2Output: 2Note: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].
本题是为了求连续数组和为k的个数。本题需要注意的是可能存在负数。首先第一种方法暴力搜索法,两个循环,但是由于负数的存在不能在碰到一个之后就break,代码如下所示:
public int subarraySum1(int[] nums, int k) { int count=0; for(int i=0;i<nums.length;i++){ int sum=nums[i]; if(sum==k){ count++; } for(int j=i+1;j<nums.length;j++){ sum+=nums[j]; if(sum==k) count++; } } return count; }
另外一种思路就是使用map来保存当前数组的求和值,然后每到一个数都将res进行更新。代码入下:
public int subarraySum(int[] nums, int k) { int sum=0, count=0; Map<Integer, Integer> map = new HashMap<>(); map.put(0, 1); for(int i=0; i<nums.length; i++){ sum += nums[i]; if(map.containsKey(sum-k)) count += map.get(sum-k); map.put(sum, map.getOrDefault(sum, 0)+1); } return count; }
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