[doh6077] WEEK 09 Solutions

linked-list-cycle/doh6077.py

@@ -0,0 +1,24 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+'''
+# 141. Linked List Cycle
+
+1. Create an empty list to keep track of visited nodes.
+2. Traverse the linked list starting from the head node.    
+
+'''
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool: 
+        record = []
+        if head == None:
+            return False 
+
+        while head.next:
+            record.append(head)
+            if head.next in record:
+                return True 
+            head = head.next 
+        return False 

maximum-product-subarray/doh6077.py

@@ -0,0 +1,25 @@
+#152. Maximum Product Subarray
+
+# https://leetcode.com/problems/maximum-product-subarray/
+# 
+
+
+# Three cases to consider 
+# negative number
+# positive number
+# zero - we reset the curMin and curMax to 1 ( neutral element for multiplication)
+class Solution:
+    def maxProduct(self, nums: List[int]) -> int:
+        res = max(nums)
+
+        curMin, curMax = 1, 1
+
+        for num in nums:
+            if num == 0 :
+                curMin, curMax = 1, 1
+                continue
+            temp = curMax * num
+            curMax = max(curMax * num, curMin * num, num)
+            curMin = min(curMin * num, temp, num)
+            res = max(res, curMax)
+        return res

pacific-atlantic-water-flow/doh6077.py

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+# # 417. Pacific Atlantic Water Flow
+from collections import deque
+
+class Solution:
+    # Time Complexity O(m*n)
+
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        #Two BFS 
+        # intersection of two hash sets 
+        p_que = deque()
+        p_seen = set()
+
+        a_que = deque()
+        a_seen = set()
+
+        m, n = len(heights), len(heights[0])
+        for j in range(n):
+            p_que.append((0,j))
+            p_seen.add((0,j))
+
+        for i in range(1,m):
+            p_que.append((i,0))
+            p_seen.add((i,0))
+
+        for i in range(m):
+            a_que.append((i, n-1))
+            a_seen.add((i, n-1))
+
+        for j in range(n-1):
+            a_que.append((m-1,j))
+            a_seen.add((m-1,j))
+
+        def get_coords(que,seen):
+            coords = set()
+            while que:
+                i, j = que.popleft()
+                coords.add((i,j))
+                for i_off, j_off in [(0,1), (1,0), (-1,0), (0,-1)]:
+                    r, c = i+i_off, j +j_off
+                    if 0 <= r < m and 0 <= c < n and heights[r][c] >= heights[i][j] and (r,c) not in seen:
+                        seen.add((r,c))
+                        que.append((r,c))
+            return coords
+
+        p_coords = get_coords(p_que, p_seen)
+        a_coords = get_coords(a_que, a_seen)
+        return list(p_coords.intersection(a_coords))
+
+# # First Draft 
+
+# class Solution:
+#     def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+
+#         # first Attempt 
+#         self.res = [] 
+#         self.pacific = []
+#         self.atlantic = []
+#         # Brute Force
+#         # DFS - every single position 
+#         # 1. Pacific Ocean ( in a row: the left side is less than the number) or ( in a column, the previous value is less than the number)
+#         # 2. Atlantic Ocean ( in a row: the right side is less than the number) or (in a column, the next values are less than the number)
+#         # 3. if the number satisfies both then we add the coordinates of the number to the result 
+
+#         def dfs(list_check):
+#             find_max = max(list_check)
+#             max_index = list_check.index(find_max)
+
+
+#             # pacific 
+#             if max_index != 0: 
+#                 check_paci = [0, max_index]
+#                 self.pacific.append(check_paci)
+#             if max_index != len(list_check):
+#                 check_atlantic = [max_index, 0]
+#                 self.atlantic.append(check_atlantic)
+
+#         if len(heights) == 1:
+#             return [[0,0]]
+
+#         for i, row in enumerate(heights):
+
+#             dfs(row)
+
+#         columns = list(zip(*heights))  # each column becomes a tuple
+#         for c, col in enumerate(columns):
+#             dfs(col)
+#         print(self.pacific)
+#         print(self.atlantic)
+
+#         return self.res 

sum-of-two-integers/doh6077.java

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+class Solution {
+    public int getSum(int a, int b) {
+        while (b != 0) {
+            int tmp = (a & b) << 1;
+            a = a ^ b;
+            b = tmp;
+        }
+        return a;
+
+    }
+}