Leetcode 167

Author: Kondareddy1209

Leetcode/Leetcode_167.py

@@ -0,0 +1,151 @@
+"""
+Problem: Two Sum
+---------------------------------
+Given an array nums and a target,
+return indices of two numbers such that:
+
+nums[i] + nums[j] = target
+
+---------------------------------
+This file contains TWO approaches:
+
+1. Two Pointer (only works for SORTED array)
+2. HashMap (works for ANY array) ✅ Optimal
+
+---------------------------------
+"""
+
+from typing import List
+
+
+class Solution:
+
+    # =========================================================
+    # 🔹 APPROACH 1: TWO POINTER (SORTED ARRAY ONLY)
+    # =========================================================
+    def twoSum_two_pointer(self, nums: List[int], target: int) -> List[int]:
+        """
+        ⚠️ IMPORTANT:
+        This works ONLY if the array is SORTED
+
+        Step-by-step:
+        1. Start with two pointers:
+           l = 0 (start), r = n-1 (end)
+        2. Calculate sum
+        3. If sum == target → return indices
+        4. If sum < target → move left pointer forward
+        5. If sum > target → move right pointer backward
+        """
+
+        l = 0
+        r = len(nums) - 1
+
+        while l < r:
+            s = nums[l] + nums[r]
+
+            if s == target:
+                return [l, r]  # 0-based indexing
+
+            elif s < target:
+                l += 1  # increase sum
+
+            else:
+                r -= 1  # decrease sum
+
+        return [-1, -1]
+
+
+    # =========================================================
+    # 🔹 APPROACH 2: HASHMAP (OPTIMAL)
+    # =========================================================
+    def twoSum_hashmap(self, nums: List[int], target: int) -> List[int]:
+        """
+        Step-by-step:
+        1. Create a hashmap to store value → index
+        2. For each number:
+            - Compute complement = target - current number
+            - Check if complement exists in hashmap
+        3. If yes → return indices
+        4. Else → store current number
+        """
+
+        hashmap = {}
+
+        for i, num in enumerate(nums):
+
+            complement = target - num
+
+            # Check if complement already exists
+            if complement in hashmap:
+                return [hashmap[complement], i]
+
+            # Store current number
+            hashmap[num] = i
+
+        return [-1, -1]
+
+
+# =========================================================
+# 🧩 DRY RUN
+# =========================================================
+"""
+Example:
+nums = [2, 7, 11, 15]
+target = 9
+
+HashMap Approach:
+-----------------
+i = 0 → num = 2 → complement = 7 → not found → store {2:0}
+i = 1 → num = 7 → complement = 2 → found → return [0,1]
+
+Two Pointer (sorted):
+---------------------
+l = 0, r = 3 → 2 + 15 = 17 > 9 → r--
+l = 0, r = 2 → 2 + 11 = 13 > 9 → r--
+l = 0, r = 1 → 2 + 7 = 9 → return [0,1]
+"""
+
+
+# =========================================================
+# ⏱️ TIME & SPACE COMPLEXITY
+# =========================================================
+"""
+1. Two Pointer:
+   --------------
+   Time Complexity: O(n)
+   Space Complexity: O(1)
+
+   ⚠️ Only works if array is sorted
+
+
+2. HashMap:
+   ----------
+   Time Complexity: O(n)
+   Space Complexity: O(n)
+
+   ✅ Works for unsorted arrays (BEST choice)
+"""
+
+
+# =========================================================
+# 🔥 KEY TAKEAWAYS
+# =========================================================
+"""
+👉 Two Pointer = only for sorted arrays
+👉 HashMap = best for general case
+👉 Avoid l <= r (use l < r)
+👉 Don't return +1 unless problem asks 1-based index
+"""
+
+
+# =========================================================
+# ✅ DRIVER CODE
+# =========================================================
+if __name__ == "__main__":
+    sol = Solution()
+
+    nums = [2, 7, 11, 15]
+    target = 9
+
+    print("Two Pointer:", sol.twoSum_two_pointer(nums, target))
+    print("HashMap:", sol.twoSum_hashmap(nums, target))