Add implementation for leetcode problems 409, 525, 560

leetcode409.py

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+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+Created on Tue Feb 10 21:21:02 2026
+
+@author: rishigoswamy
+
+
+    Problem: Longest Palindrome
+    Link: https://leetcode.com/problems/longest-palindrome/
+
+    Description:
+    Given a string s consisting of uppercase and lowercase letters,
+    return the length of the longest palindrome that can be built
+    with those letters.
+
+    Approach:
+    Use a HashSet to track characters with odd frequency.
+
+    - If a character appears twice, it contributes +2 to length.
+    - Characters left in the set at the end have odd frequency.
+    - We can place exactly one odd-frequency character in the center.
+
+    Time Complexity: O(n)
+    Space Complexity: O(1)  # At most 52 characters (A-Z, a-z)
+    """
+
+class Solution:
+    def longestPalindrome(self, s: str) -> int:
+        hSet = set()
+        length = 0
+        for char in s:
+            if char in hSet:
+                length += 2
+                hSet.remove(char)
+            else:
+                hSet.add(char)
+
+        return length + (1 if len(hSet) > 0 else 0)

leetcode525.py

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+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+Created on Tue Feb 10 21:19:45 2026
+
+@author: rishigoswamy
+
+    Problem: Contiguous Array
+    Link: https://leetcode.com/problems/contiguous-array/
+
+    Description:
+    Given a binary array nums, return the maximum length 
+    of a contiguous subarray with an equal number of 0 and 1.
+
+    Approach:
+    Convert 0 → -1.
+    Then the problem becomes:
+    Find the longest subarray with sum = 0.
+
+    Use Prefix Sum + HashMap to store the first occurrence 
+    of each prefix sum.
+
+    Time Complexity: O(n)
+    Space Complexity: O(n)
+
+
+"""
+
+from typing import List
+
+class Solution:
+    def findMaxLength(self, nums: List[int]) -> int:
+        prefixSum = 0
+        hMap = {}
+        hMap[0] = -1
+        res = 0
+        for i in range(len(nums)):
+            prefixSum += 1 if nums[i] == 1 else -1
+
+            if prefixSum in hMap:
+                res = max(res , i - hMap[prefixSum])
+            else:
+                hMap[prefixSum] = i
+        return res  

leetcode560.py

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+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+Created on Tue Feb 10 21:17:29 2026
+
+@author: rishigoswamy
+
+    Problem: Subarray Sum Equals K
+    Link: https://leetcode.com/problems/subarray-sum-equals-k/
+
+    Approach:
+    Use Prefix Sum + HashMap to store frequency of prefix sums.
+
+    Time Complexity: O(n)
+    Space Complexity: O(n)
+
+"""
+
+from typing import List
+
+class Solution:
+    def subarraySum(self, nums: List[int], k: int) -> int:
+        count = 0
+        hMap = {}
+        prefixSum = 0
+
+        for item in nums:
+            prefixSum += item
+            if prefixSum == k:
+                count+=1
+            if prefixSum - k in hMap:
+                count+= hMap[prefixSum - k]
+            if prefixSum not in hMap:
+                hMap[prefixSum] = 1
+            else:
+                hMap[prefixSum]+= 1
+
+        return count