LeetCode 560. Subarray Sum Equals K

原题链接在这里：https://leetcode.com/problems/subarray-sum-equals-k/description/

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 = 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].

题解：

计算subarray的和为k的个数.

保存从头到当前值和sum 以及 对应这个和的个数. 若是之前出现了sum-k说明这一段相加就是k. 更新和.

Note: 初始HashMap<Integer, Integer> hm 时需要先把和为0的情况加进去. 最开始什么都没有, 所以hm需要加上<0,1>这个对应关系.

Time Complexity: O(nums.length).

Space: O(nums.length).

AC Java:

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

类似Two Sum, Continuous Subarray Sum, Subarray Product Less Than K, Find Pivot Index.