vault backup: 2022-09-05 15:36:37

.obsidian/graph.json

@@ -39,6 +39,6 @@
   "repelStrength": 10,
   "linkStrength": 1,
   "linkDistance": 250,
-  "scale": 0.7878344292720599,
+  "scale": 0.7463733455586773,
   "close": true
 }

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

@@ -0,0 +1,103 @@
+# 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](https://leetcode.com/problems/subarray-sum-equals-k/)
+
+---
+
+### 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
+```cpp
+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;
+  }
+};
+```