progress: Hashing-2 implemented

ContiguousSubarray.cs

@@ -0,0 +1,43 @@
+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+
+// Treat 0 as -1 and 1 as +1, and maintain a running sum; equal numbers of 0s and 1s result in the same prefix sum repeating.
+// Use a dictionary to store the first index where each prefix sum occurs (initialize sum = 0 at index -1).
+// When the same sum appears again, the subarray between the previous index and current index has equal 0s and 1s; 
+// update the maximum length.
+public class Solution
+{
+    public int FindMaxLength(int[] nums)
+    {
+        if (nums == null || nums.Length == 0)
+            return 0;
+        Dictionary<int, int> dict = new();
+        int max = 0; int sum = 0;
+        dict.Add(0, -1);
+        for (int i = 0; i < nums.Length; i++)
+        {
+            if (nums[i] == 0)
+                sum -= 1;
+            else
+                sum += 1;
+
+            if (dict.ContainsKey(sum))
+            {
+                int currentIndex = i - dict[sum];
+                max = Math.Max(max, currentIndex);
+                // if you need  maintain indexes
+                // if (max < currentIndex)
+                // {
+                //     int start = dict[sum] + 1;
+                //     int end = i;
+                // }
+            }
+            else
+            {
+                dict.Add(sum, i);
+            }
+        }
+        return max;
+    }
+}

longestPalindrome.cs

@@ -0,0 +1,32 @@
+//Time and space - O(n)
+
+// Use a HashSet to track characters that appear an odd number of times.
+// When a character appears twice, form a pair, add 2 to the palindrome length, and remove it from the set.
+// After processing all characters, if any odd-count character remains, add 1 to place one in the center
+public class Solution
+{
+    public int LongestPalindrome(string s)
+    {
+        HashSet<char> hash = new();
+        int count = 0;
+        for (int i = 0; i < s.Length; i++)
+        {
+            char ch = s[i];
+            if (hash.Contains(ch))
+            {
+                count += 2;
+                hash.Remove(ch);
+            }
+            else
+            {
+                hash.Add(ch);
+            }
+        }
+        if (hash.Count != 0)
+        {
+            count += 1;
+        }
+        return count;
+    }
+}
+

subArraySumEqualsK.cs

@@ -0,0 +1,35 @@
+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+// Use a prefix sum (rsum) and a dictionary to store how many times each prefix sum has appeared.
+// For each index, check if (rsum − k) exists in the dictionary; if it does, those occurrences form subarrays with sum k.
+// Update the dictionary with the current prefix sum count as you iterate.
+public class Solution
+{
+    public int SubarraySum(int[] nums, int k)
+    {
+        Dictionary<int, int> dict = new();
+        int rsum = 0;
+        int count = 0;
+        dict.Add(0, 1);
+        for (int i = 0; i < nums.Length; i++)
+        {
+            rsum += nums[i];
+            int res = rsum - k;
+            if (dict.ContainsKey(res))
+            {
+                count += dict[res];
+            }
+            if (dict.ContainsKey(rsum))
+            {
+                dict[rsum]++;
+            }
+            else
+            {
+                dict.TryAdd(rsum, 1);
+            }
+        }
+        return count;
+
+    }
+}