Hashing

0001_TwoSum_HashMap.java

@@ -0,0 +1,17 @@
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        HashMap<Integer, Integer> map = new HashMap<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            int complement = target - nums[i];
+
+            if (map.containsKey(complement)) {
+                return new int[] { map.get(complement), i };
+            }
+
+            map.put(nums[i], i);
+        }
+
+        return new int[] {}; 
+    }
+}

0128_Longest_Consecutive_Sequence.java

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+import java.util.HashSet;
+
+class Solution {
+    public int longestConsecutive(int[] nums) {
+
+        HashSet<Integer> set = new HashSet<>();
+
+        // Add all numbers to set
+        for (int num : nums) {
+            set.add(num);
+        }
+
+        int longest = 0;
+
+        for (int num : set) {
+
+            // Check if it's the start of a sequence
+            if (!set.contains(num - 1)) {
+
+                int currentNum = num;
+                int currentStreak = 1;
+
+                while (set.contains(currentNum + 1)) {
+                    currentNum++;
+                    currentStreak++;
+                }
+
+                longest = Math.max(longest, currentStreak);
+            }
+        }
+
+        return longest;
+    }
+}

README.md

@@ -1,3 +1,23 @@
+# Hashing
+
+## 1. Two Sum
+Problem Link: https://leetcode.com/problems/two-sum/
+Difficulty: Easy
+### Approach : HashMap (Optimized)
+Time Complexity: O(n)
+Space Complexity: O(n)
+### Concepts Used:
+Array Traversal,HashMap,Complement Technique,Single Pass Algorithm,Key-Value Mapping
+
+## 128. Longest Consecutive Sequence
+Problem Link: https://leetcode.com/problems/longest-consecutive-sequence/
+Difficulty: Medium
+### Approach: HashSet (Optimized)
+Time Complexity: O(n)
+Space Complexity: O(n)
+### Concepts Used:
+HashSet, Sequence Pattern, Array Traversal
+=======
 # Prefix-Sum
 
 ## 523. Continuous Subarray Sum