notes/OJ notes/pages/Leetcode Subarray-Sum-Equals-K.md

Leetcode Subarray-Sum-Equals-K

2022-09-05 15:22

Algorithms:

#algorithm #sliding_window #hash_table

Data structures:

#DS

Difficulty:

#coding_problem #difficulty-medium

Additional tags:

#leetcode

Revisions:

N/A

Links:

• Link to problem

---

Problem

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

A subarray is a contiguous non-empty sequence of elements within an array.

Examples

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

Thoughts

[!summary] This can be solved using #sliding_window

I over-complicated the solution by adding stuff like sorting, but it turn out to be not so difficult.

Since the subsets are contiguous, we can use sliding window here.

Solution

TLE, brute force

class Solution {
public:
  int subarraySum(vector<int> &nums, int k) {
    // O(N^2 / 2)

    int n = nums.size();

    if (n == 1) {
      return (k == nums[0]);
    }

    vector<int> prefix(n + 1, 0);
    int prefixSum = 0;

    for (int i = 0; i < n; i++) {
      prefixSum += nums[i];
      prefix[i + 1] = prefixSum;
    }

    int count = 0;

    for (int i = 0; i < n + 1; i++) {
      for (int j = i + 1; j < n + 1; j++) {
        if (prefix[j] - prefix[i] == k) {
          count++;
        }
      }
    }

    return count;
  }
};