Two-Pointers-1

3sum.py

@@ -0,0 +1,41 @@
+"""
+time - n^2(log(n))
+space - o(1)
+we sort the array, we take three pointers, one main pointer that iterates through the array, another two where left starts
+from main pointer +1 and right starts at the end. We take the complement as 0-nums[i] and using the left and right pointers
+we find the two sum solution for the compliment.
+"""
+
+from typing import List
+
+
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        nums.sort()
+        print(nums)
+        result = []
+        for i in range(len(nums)):
+            if nums[i] > 0:
+                break
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+            two_sum = 0 - nums[i]
+            l = i + 1
+            r = len(nums) - 1
+            while l < r:
+                left = nums[l]
+                right = nums[r]
+                temp_sum = left + right
+                if temp_sum == two_sum:
+                    result.append([nums[i], nums[l], nums[r]])
+                    while l < r and nums[l] == left:
+                        l += 1
+                    while l < r and nums[r] == right:
+                        r -= 1
+                elif temp_sum > two_sum:
+                    while l < r and nums[r] == right:
+                        r -= 1
+                else:
+                    while l < r and nums[l] == left:
+                        l += 1
+        return result

container_water.py

@@ -0,0 +1,22 @@
+"""
+time - o(n)
+space - o(1)
+we use two pointers that start at the ends, we calculate the area by multiplying distance between two pointers and least height among
+the two. We take a greedy approach by moving the smaller of the two heights
+"""
+
+from typing import List
+
+
+class Solution:
+    def maxArea(self, height: List[int]) -> int:
+        left = 0
+        right = len(height) - 1
+        maxArea = float("-inf")
+        while left < right:
+            maxArea = max(maxArea, min(height[left], height[right]) * (right - left))
+            if height[left] <= height[right]:
+                left += 1
+            else:
+                right -= 1
+        return maxArea

dutchflag.py

@@ -0,0 +1,26 @@
+"""
+time - o(n)
+space - o(1)
+we take three pointers, one to iterate and two as place holders, left and right are placeholder pointers that keep a track of 0s and 2s
+as and when the third pointer encounters a 0 or 2, we swap it with the left and right pointers, if it comes across a one we move the pointers
+"""
+
+from typing import List
+
+
+class Solution:
+    def sortColors(self, nums: List[int]) -> None:
+        """
+        Do not return anything, modify nums in-place instead.
+        """
+        l, r, m = 0, len(nums) - 1, 0
+        while m <= r:
+            if nums[m] == 0:
+                nums[l], nums[m] = nums[m], nums[l]
+                l += 1
+                m += 1
+            elif nums[m] == 2:
+                nums[m], nums[r] = nums[r], nums[m]
+                r -= 1
+            else:
+                m += 1