completed 2-pointers

probelm34.py

@@ -0,0 +1,29 @@
+class Solution:
+    def sortColors(self, nums: List[int]) -> None:
+        """
+        Do not return anything, modify nums in-place instead.
+        """
+        left = 0 #to catch 0s
+        right = len(nums)-1 #to catch 2s
+        mid = left #to iterate the array and to catch 1s
+
+        while mid <= right:
+            if nums[mid] == 2:
+                nums[mid], nums[right] = nums[right], nums[mid]
+                right -= 1 #cant move mid here because left is not yet sorted. The value swapped from the right into mid is unknown. It can be 0,1,2 so we need to re-check it in next iteration.
+
+            elif nums[mid] == 0:
+                nums[mid], nums[left] = nums[left], nums[mid]
+                left += 1
+                mid += 1 #left is sorted so move mid. everything before mid was already processed, so the swapped value is guaranteed to be a 1. it can't be a 2 as its always taken care of first.
+            else:
+                mid += 1
+
+# You only move mid when you are sure the value at mid is finalized.
+
+# After 0 → finalized
+# After 1 → finalized
+# After swapping with right → not finalized
+
+# Time complexity: O(N)
+# Space Complexity: O(1)

problem35.py

@@ -0,0 +1,84 @@
+from typing import List
+
+#solution 1: hashing based
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        #hashing based solution
+        nums.sort()
+
+        result = set()
+
+        for i in range(len(nums)):
+            if i != 0 and nums[i] == nums[i-1]:#to avoid duplicates iterate till u find next unique
+                continue
+
+            inner_sum = -nums[i]
+            seen = set()
+            for j in range(i+1,len(nums)):
+                compliment = inner_sum - nums[j]
+
+                if compliment in seen:
+                    triplet = sorted([compliment, nums[j], nums[i]]) #O(3) 
+                    result.add(tuple(triplet))
+                seen.add(nums[j])
+
+        return [list(triplet) for triplet in result]
+
+# You only skip duplicates after a valid triplet
+# You never skip a value that could form a new combination, so can't add duplicate check condition for inner j loop. Use sorting here.
+# Order guarantees no missed cases
+
+# Time Complexity: O(nlogn) + O(n2) + O(n) = O(n2)
+# space complexity: O(n)
+
+#Solution 2 : 2 pointer based
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        #2 pointer based solution
+        nums.sort()
+
+        result = []
+
+        for i in range(len(nums)):
+            if i != 0 and nums[i] == nums[i-1]:
+                continue
+            if nums[i] > 0:
+                break #early stopping as we are already sorted the array in asc order so elements after this will always be larger and
+                #we won't get sum = 0.
+
+            left = i+1
+            right = len(nums)-1
+            inner_sum = -nums[i]
+
+            while left < right:
+                if nums[left]+nums[right] == inner_sum:
+                    result.append([nums[i], nums[left], nums[right]])
+                    left += 1
+                    right -= 1
+                    while (left < right and nums[left] == nums[left-1]):
+                        left += 1
+
+                    while (left<right and nums[right] == nums[right+1]):
+                        right -= 1
+
+                elif nums[left]+nums[right] > inner_sum:
+                    right -=1
+                    while (left<right and nums[right] == nums[right+1]):
+                        right -= 1
+
+                else:
+                    left += 1
+                    while (left < right and nums[left] == nums[left-1]):
+                        left += 1
+
+
+                #to handle inner duplicacy. we can also check if array is going OOB
+                #but best way to do is by checking the base condition again
+                #if base condition variables have been mutated.
+
+        return result
+
+
+# You only skip duplicates after a valid triplet if found
+# Time Complexity: O(nlogn) + O(n2) = O(n2)
+# space complexity: O(1)

problem36.py

@@ -0,0 +1,37 @@
+#limiting factors are width and min(h1,h1), the min height. So we start with max width possible (two-pointer greedy approach) and the
+#move the pointers accordingly to maximize the area possible.
+#after we have explored the max water possible with a pointer (left or right), we move it.
+
+from typing import List
+
+
+class Solution:
+    def maxArea(self, height: List[int]) -> int:
+
+        max_area = 0
+
+        left , right = 0, len(height)-1
+
+        while left <= right: #it can be left < right also
+            area = min(height[left], height[right]) * (right-left)
+
+            max_area = max(area, max_area)
+
+            if height[left] < height[right]:
+                left += 1
+            else:
+                right -=1
+
+        return max_area
+
+# Time Complexity: O(N)
+# Space Complexity: O(1)
+#brute force is nested iterations O(N2)
+# The two-pointer approach does not necessarily compute the maximum area for each individual height.
+# It still finds the global maximum, because it only evaluates the pairs that could possibly beat the best-so-far.
+# example left = 2, right = 8 width = 50
+# min(2,8) * 50 = 2*50 = 100
+# we need to beat this 100. The only way we can beat 100 is to move left pointer, if left pointer is some value say 8 
+# then min(8,8)= 8*49
+# if we move right pointer max we can get is min(2,2)*49. which is 2*49 at best we can find.
+#Two pointers rely on discarding indices that can’t be part of the global optimum.