completed 2-pointers

problem37.py

@@ -0,0 +1,37 @@
+class Solution:
+    def merge(self, nums1: List[int], m: int, nums2: List[int], n: int) -> None:
+        """
+        Do not return anything, modify nums1 in-place instead.
+        """
+        first = m-1
+        second = n-1
+        curr = len(nums1)-1
+
+        while first >= 0 and second >= 0:#stop when either array is exhausted
+            if nums1[first] > nums2[second]:
+                nums1[curr] = nums1[first]
+                first -= 1
+                curr -= 1
+            else:
+                nums1[curr] = nums2[second]
+                second -= 1
+                curr -= 1
+
+        if first < 0:#first array got exhausted, then copy second to first. Meaning we processed all elements in first array to their correct position
+            while second >= 0:
+                nums1[curr] = nums2[second]
+                curr -= 1
+                second -=1
+
+        #if second array got exhausted do nothing. as it will be correctly placed to the end of the nums1 array.
+
+
+#if we start from the begining of array, there is a chance we may lose the sorting
+#[5 6 7 0 0 0] [2 3 4] -> second array can become [5 3 4], we are losing sorting
+# so start from where there is no issue of overwriting. we won't lose sorted order. so
+# collect and put to the last. Collecting pointer from last.
+# Collecting point shud start from the end, if it stars from the begining the we can  lose sorting.
+# comparing pointers start from the end as well bcuz bigger no can go to the end.  
+
+# Time Complexity: O(m+n)
+# Space Complexity: O(1)

problem38.py

@@ -0,0 +1,33 @@
+class Solution:
+    def searchMatrix(self, matrix: List[List[int]], target: int) -> bool:
+        #here ful 2D matrix is not sorted. Only rows are sorted.
+        #so can't consider as a 1-D array.
+        #we use steps method O(m+n)
+        #can do BS on rows O(m*logn)
+        #can do BS on column O(n*logm)
+        #brute force is O(m*n)
+
+        #in steps method start from top right or bottom left cell.
+        #other two corners we can't make a decision in which way to proceed as both sides will be either smaller or greater
+
+        #starting from top right corner here
+
+        m = len(matrix)
+        n = len(matrix[0])
+
+        #right most corner, for left bottom take accordingly
+        row = 0
+        col = n-1
+
+        while 0 <= row < m and 0 <= col < n:
+            if target == matrix[row][col]:
+                return True
+            elif target > matrix[row][col]:
+                row += 1
+            else:
+                col -= 1
+
+        return False
+
+        # Time Complexity: O(m+n)
+        # Space Complexity : O(1)

problem39.py

@@ -0,0 +1,31 @@
+class Solution:
+    def removeDuplicates(self, nums: List[int]) -> int:
+        #brute force solution
+        #create a freq map, then overwrite the array but O(n) space
+
+        slow = 0 #this is to re-write the array
+        fast = 0 #this is to iterate over the array
+
+        count = 0
+
+        while fast < len(nums):
+
+            curr = nums[fast]
+
+            while (fast < len(nums) and nums[fast] == curr):
+                fast += 1
+                count += 1
+
+            jumps = min(count, 2)
+
+            while jumps > 0:
+                nums[slow] = curr
+                slow += 1
+                jumps -= 1
+
+            count = 0
+
+        return slow
+
+    # Time Complexity: O(2N) = O(N)
+    # Space Complexity: O(1)