From 7aa99004fc6934ac9876de9a91105ef42cdba4bd Mon Sep 17 00:00:00 2001
From: nikhylw <1.nikhil.wani+nikhly@gmail.com>
Date: Mon, 4 May 2026 22:30:43 +0530
Subject: [PATCH] add hashing-2 solutions (all 3)

---
 W2_3_409_longest_palindrome.py    | 22 ++++++++++++++++++++++
 W2_4_525_contiguous_array.py      | 27 +++++++++++++++++++++++++++
 W2_5_560_subarray_sum_equals_k.py | 22 ++++++++++++++++++++++
 3 files changed, 71 insertions(+)
 create mode 100644 W2_3_409_longest_palindrome.py
 create mode 100644 W2_4_525_contiguous_array.py
 create mode 100644 W2_5_560_subarray_sum_equals_k.py

diff --git a/W2_3_409_longest_palindrome.py b/W2_3_409_longest_palindrome.py
new file mode 100644
index 00000000..8ea143c4
--- /dev/null
+++ b/W2_3_409_longest_palindrome.py
@@ -0,0 +1,22 @@
+# Time complexity: O(N), where n is the length of characters in the string
+# Space complexity: O(1)
+
+
+class Solution:
+    def longestPalindrome(self, s: str) -> int:
+        hashset = set()
+        count = 0 
+
+        for i in range(0, len(s)):
+            ch = s[i]
+
+            if ch in hashset:
+                count = count + 2
+                hashset.remove(ch)
+            else:
+                hashset.add(ch)
+        
+        if len(hashset) != 0:
+            count = count + 1
+        
+        return count
diff --git a/W2_4_525_contiguous_array.py b/W2_4_525_contiguous_array.py
new file mode 100644
index 00000000..a506a2d7
--- /dev/null
+++ b/W2_4_525_contiguous_array.py
@@ -0,0 +1,27 @@
+#Time complexity: O(N)
+#Space complexity: O(N)
+
+class Solution:
+    def findMaxLength(self, nums: List[int]) -> int:
+
+        hashmap = {}
+        maxx = 0 
+        rsum = 0 
+
+        hashmap[0] = -1
+
+        for i in range(len(nums)):
+            if nums[i] == 0:
+                rsum = rsum - 1
+            else:
+                rsum = rsum + 1
+
+            if rsum not in hashmap:
+                hashmap[rsum] = i
+            else:
+                curr = i - hashmap[rsum]
+                maxx = max(maxx, curr)
+
+        return maxx
+
+
diff --git a/W2_5_560_subarray_sum_equals_k.py b/W2_5_560_subarray_sum_equals_k.py
new file mode 100644
index 00000000..4dc7c12b
--- /dev/null
+++ b/W2_5_560_subarray_sum_equals_k.py
@@ -0,0 +1,22 @@
+#Time complexity: O(n)
+#Space complexity: O(n)
+
+class Solution:
+    def subarraySum(self, nums: List[int], k: int) -> int:
+        hashmap = dict()
+        hashmap[0] = 1
+        rsum = 0
+        count = 0
+
+        for i in range(0, len(nums)):
+            rsum = rsum + nums[i]
+            
+            if rsum - k in hashmap:
+                count = count + hashmap[rsum - k]
+            
+            if rsum not in hashmap:
+                hashmap[rsum] = 0
+
+            hashmap[rsum] = hashmap[rsum] + 1
+        
+        return count