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leetcode_java/src/main/java/LeetCodeJava/HashTable/SubarraySumEqualsK.java
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| package LeetCodeJava.HashTable; | ||
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| // https://leetcode.com/problems/subarray-sum-equals-k/description/ | ||
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| import java.util.HashMap; | ||
| import java.util.Map; | ||
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| /** | ||
| * 560. Subarray Sum Equals K | ||
| * Solved | ||
| * Medium | ||
| * Topics | ||
| * Companies | ||
| * Hint | ||
| * 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. | ||
| * | ||
| * | ||
| * | ||
| * 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 | ||
| * Seen this question in a real interview before? | ||
| * 1/5 | ||
| * Yes | ||
| * No | ||
| * Accepted | ||
| * 1.5M | ||
| * Submissions | ||
| * 3.4M | ||
| * Acceptance Rate | ||
| * 44.5% | ||
| * Topics | ||
| */ | ||
| public class SubarraySumEqualsK { | ||
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| // V0 | ||
| // IDEA : HASH MAP (fixed by gpt) | ||
| /** | ||
| * | ||
| * Explanation of Fixes: | ||
| * | ||
| * 1. Using presum - k Logic Properly: | ||
| * • If presum - k exists in the map, it means there is a subarray ending at the current index whose sum is k . Add the count of such prefix sums to the total. | ||
| * | ||
| * 2. Map Initialization: | ||
| * • Initializing the map with map.put(0, 1) allows the logic to handle cases where a subarray starting from index 0 equals k . | ||
| * | ||
| * 3. Updating the Map: | ||
| * • Instead of tracking indices, the map stores the count of occurrences of each prefix sum. This allows us to find how many subarrays can be formed using a specific prefix sum. | ||
| * | ||
| * 4. Simplifying Index-Based Logic: | ||
| * • Removed unnecessary index-based conditions (map.get(presum - k) == i + 1) which were incorrect and not required for this problem. | ||
| */ | ||
| public int subarraySum(int[] nums, int k) { | ||
| // Map to store prefix sum and its count | ||
| Map<Integer, Integer> map = new HashMap<>(); | ||
| int presum = 0; | ||
| int count = 0; | ||
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| // Initialize the map with prefix sum 0 (to handle subarrays starting at index 0) | ||
| map.put(0, 1); | ||
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| for (int num : nums) { | ||
| presum += num; | ||
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| // Check if there's a prefix sum such that presum - k exists | ||
| if (map.containsKey(presum - k)) { | ||
| count += map.get(presum - k); | ||
| } | ||
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| // Update the map with the current prefix sum | ||
| map.put(presum, map.getOrDefault(presum, 0) + 1); | ||
| } | ||
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| return count; | ||
| } | ||
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| // V1 | ||
| // IDEA : HASH MAP | ||
| // https://leetcode.com/problems/subarray-sum-equals-k/solutions/6143642/java-beats-9983-by-mohamedhazem3-9yj6/ | ||
| public static int subarraySum_1(int[] nums, int k) { | ||
| int[] prefix = new int[nums.length + 1]; | ||
| int out = 0; | ||
| HashMap<Integer, Integer> hm = new HashMap<>(); | ||
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| for (int i = 0; i < nums.length; i++) | ||
| prefix[i + 1] = prefix[i] + nums[i]; // Compute prefix sums | ||
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| for (int i = 0; i < prefix.length; i++) { | ||
| if (hm.containsKey(prefix[i] - k)) // Check if required difference exists | ||
| out += hm.get(prefix[i] - k); // Add the frequency to the result | ||
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| // Update HashMap with the current prefix sum | ||
| hm.put(prefix[i], hm.getOrDefault(prefix[i], 0) + 1); | ||
| } | ||
| return out; | ||
| } | ||
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| // V2 | ||
| // IDEA : HASH MAP | ||
| // https://leetcode.com/problems/subarray-sum-equals-k/solutions/3167609/explained-beats-964-using-hashmap-in-jav-0b6o/ | ||
| public int subarraySum_2(int[] nums, int k) { | ||
| int sum = 0; | ||
| int ans = 0; | ||
| HashMap<Integer,Integer> map = new HashMap<>(); | ||
| map.put(0,1); | ||
| for(int j=0;j<nums.length;j++){ | ||
| sum += nums[j]; | ||
| if(map.containsKey(sum -k)){ | ||
| ans += map.get(sum-k); | ||
| } | ||
| map.put(sum,map.getOrDefault(sum,0)+1); | ||
| } | ||
| return ans; | ||
| } | ||
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| // V3 | ||
| // IDEA : HASH MAP | ||
| // https://leetcode.com/problems/subarray-sum-equals-k/solutions/803317/java-solution-with-detailed-explanation-yu2hg/ | ||
| public int subarraySum_3(int[] nums, int k) { | ||
| int count = 0; | ||
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| int[] sum = new int[nums.length + 1]; | ||
| sum[0] = 0; | ||
| for (int i = 1; i <= nums.length; i++) | ||
| sum[i] = sum[i - 1] + nums[i - 1]; | ||
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| for (int start = 0; start < sum.length; start++) { | ||
| for (int end = start + 1; end < sum.length; end++) { | ||
| if (sum[end] - sum[start] == k) | ||
| count++; | ||
| } | ||
| } | ||
| return count; | ||
| } | ||
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| } |
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