Update solutions

Author: robinali34

README.md

@@ -0,0 +1,28 @@
+#! /usr/bin/python3
+# -*- coding:utf8 -*-
+
+# Copyright 2025 Robina Li. BSD 3-Clause License All Rights Reserved.
+# @file : solution.py
+# @desc : Solution for Leetcode
+# Reference : https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
+
+from typing import List
+import math
+
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        min_, profit = float("inf"), 0
+        for i in range(len(prices)):
+            if prices[i] < min_:
+                min_ = prices[i]
+            elif prices[i] - min_ > profit:
+                profit = prices[i]- min_
+        return profit
+
+if __name__ == '__main__':
+    solution = Solution()
+
+    rtn = solution.maxProfit([7,1,5,3,6,4])
+    print(5 == rtn)
+    rtn = solution.maxProfit([7,6,4,3,1])
+    print(0 == rtn)

src/easy/__init__.py

@@ -0,0 +1,33 @@
+#! /usr/bin/python3
+# -*- coding:utf8 -*-
+
+# Copyright 2025 Robina Li. BSD 3-Clause License All Rights Reserved.
+# @file : solution.py
+# @desc : Solution for Leetcode
+# Reference : https://leetcode.com/problems/binary-search/
+
+from typing import List
+import math
+
+class Solution:
+    def search(self, nums: List[int], target: int) -> int:
+        if len(nums) == 0:
+            return -1
+        left, right = 0, len(nums) -1
+        while left <= right:
+            pivot = (left + right) // 2
+            if nums[pivot] == target:
+                return pivot
+            elif nums[pivot] < target:
+                left = pivot + 1
+            else:
+                right = pivot - 1
+        return -1
+
+if __name__ == '__main__':
+    solution = Solution()
+
+    rtn = solution.search([-1,0,3,5,9,12], 9)
+    print(4 == rtn)
+    rtn = solution.search([-1,0,3,5,9,12], 2)
+    print(-1 == rtn)

src/easy/best_time_to_buy_and_sell_stock/solution.py

@@ -15,11 +15,30 @@ def fib(self, n: int) -> int:
         if n <= 1: return n
         return self.fib(n - 1) + self.fib(n - 2)
 
+    def __init__(self):
+        self.cache = dict()
+    def fib1(self, n: int) -> int:
+        if n in self.cache:
+            return self.cache[n]
+        if n < 2:
+            rtn = n
+        else:
+            rtn = self.fib(n - 1) + self.fib(n - 2)
+        self.cache[n] = rtn
+        return rtn
+
 if __name__ == '__main__':
     solution = Solution()
     rtn = solution.fib(2)
     print(1 == rtn)
     rtn = solution.fib(3)
     print(2 == rtn)
     rtn = solution.fib(4)
+    print(3 == rtn)
+
+    rtn = solution.fib1(2)
+    print(1 == rtn)
+    rtn = solution.fib1(3)
+    print(2 == rtn)
+    rtn = solution.fib1(4)
     print(3 == rtn)

src/easy/binary_search/solution.py

@@ -0,0 +1,41 @@
+#! /usr/bin/python3
+# -*- coding:utf8 -*-
+
+# Copyright 2025 Robina Li. BSD 3-Clause License All Rights Reserved.
+# @file : solution.py
+# @desc : Solution for Leetcode
+# Reference : https://leetcode.com/problems/first-bad-version/
+
+from typing import List
+import math
+
+def isBadVersion(version: int) -> bool:
+    return version >= FIRST_BAD_VERSION
+
+class Solution:
+    def firstBadVersion(self, n: int) -> int:
+        left, right = 1, n
+        while left <= right:
+            pivit = (left + right) >> 1
+            if isBadVersion(pivit) == False:
+                left = pivit + 1
+            else:
+                right = pivit - 1
+        return left
+
+def test_first_bad_version(n, expected):
+    global FIRST_BAD_VERSION
+    FIRST_BAD_VERSION = expected  # Set the "bad" version globally
+    solution = Solution()
+    result = solution.firstBadVersion(n)
+    print(f"Input n = {n}, First Bad Version = {expected} -> Output = {result}")
+    assert result == expected, f"❌ Expected {expected}, but got {result}"
+    print("✅ Test passed\n")
+
+
+if __name__ == '__main__':
+    test_first_bad_version(5, 4)
+    test_first_bad_version(1, 1)
+    test_first_bad_version(10, 1)
+    test_first_bad_version(1000, 873)
+    test_first_bad_version(2, 2)

src/easy/fibonacci_number/solution.py

@@ -0,0 +1,66 @@
+#! /usr/bin/python3
+# -*- coding:utf8 -*-
+
+# Copyright 2025 Robina Li. BSD 3-Clause License All Rights Reserved.
+# @file : solution.py
+# @desc : Solution for Leetcode
+# Reference : https://leetcode.com/problems/search-in-a-binary-search-tree/
+
+from typing import List, Optional
+import math
+
+class TreeNode:
+    def __init__(self, val: int = 0, left: Optional['TreeNode'] = None, right: Optional['TreeNode'] = None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def searchBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
+        # Base case
+        if not root: return None
+        if root.val == val: return root
+
+        # Recursion relation
+        if val > root.val:
+            return self.searchBST(root.right, val)
+        else:
+            return self.searchBST(root.left, val)
+
+from collections import deque
+
+def build_tree_from_list(values):
+    if not values:
+        return None
+    root = TreeNode(values[0])
+    queue = deque([root])
+    i = 1
+    while queue and i < len(values):
+        node = queue.popleft()
+        if values[i] is not None:
+            node.left = TreeNode(values[i])
+            queue.append(node.left)
+        i += 1
+
+        if i < len(values) and values[i] is not None:
+            node.right = TreeNode(values[i])
+            queue.append(node.right)
+        i += 1
+    return root
+
+
+def test_search_bst(tree_vals, search_val, expected_val):
+    root = build_tree_from_list(tree_vals)
+    solution = Solution()
+    result = solution.searchBST(root, search_val)
+    output = result.val if result else None
+    print(f"Search {search_val} in {tree_vals} → Found: {output}, Expected: {expected_val}")
+    assert output == expected_val, f"❌ Failed: expected {expected_val}, got {output}"
+    print("✅ Test passed\n")
+
+if __name__ == "__main__":
+    test_search_bst([4, 2, 7, 1, 3], 2, 2)
+    test_search_bst([4, 2, 7, 1, 3], 5, None)
+    test_search_bst([], 1, None)
+    test_search_bst([1], 1, 1)
+    test_search_bst([5, 3, 6, 2, 4], 4, 4)

src/easy/first_bad_version/solution.py

@@ -0,0 +1,27 @@
+#! /usr/bin/python3
+# -*- coding:utf8 -*-
+
+# Copyright 2025 Robina Li. BSD 3-Clause License All Rights Reserved.
+# @file : solution.py
+# @desc : Solution for Leetcode
+# Reference : https://leetcode.com/problems/single-number/
+
+from typing import List
+import math
+
+class Solution:
+    def singleNumber(self, nums: List[int]) -> int:
+        a = 0
+        for i in nums:
+            a ^= i
+        return a
+
+if __name__ == '__main__':
+    solution = Solution()
+
+    rtn = solution.singleNumber([2,2,1])
+    print(1 == rtn)
+    rtn = solution.singleNumber([4,1,2,1,2])
+    print(4 == rtn)
+    rtn = solution.singleNumber([1])
+    print(1 == rtn)

src/easy/search_in_a_binary_search_tree/solution.py

@@ -0,0 +1,48 @@
+#! /usr/bin/python3
+# -*- coding:utf8 -*-
+
+# Copyright 2025 Robina Li. BSD 3-Clause License All Rights Reserved.
+# @file : solution.py
+# @desc : Solution for Leetcode
+# Reference : https://leetcode.com/problems/sqrtx/
+
+from typing import List
+from functools import cache
+from math import e, log
+
+class Solution(object):
+
+    def mySqrt(self, x: int) -> int:
+        if x < 2:
+            return x
+        left = int(e**(0.5 * log(x)))
+        right = left + 1
+        return left if right * right > x else right
+
+    def mySqrt1(self, x: int) -> int:
+        if x < 2:
+            return x
+        left, right = 2, x // 2
+        while left <= right:
+            pivot = left + (right - left) // 2
+            num = pivot * pivot
+            if num > x:
+                right = pivot - 1
+            elif num < x:
+                left = pivot + 1
+            else:
+                return pivot
+        return right
+
+if __name__ == '__main__':
+    solution = Solution()
+
+    rtn = solution.mySqrt(4)
+    print(2 == rtn)
+    rtn = solution.mySqrt(8)
+    print(2 == rtn)
+
+    rtn = solution.mySqrt1(4)
+    print(2 == rtn)
+    rtn = solution.mySqrt1(8)
+    print(2 == rtn)

src/easy/single_number/solution.py

@@ -0,0 +1,29 @@
+#! /usr/bin/python3
+# -*- coding:utf8 -*-
+
+# Copyright 2025 Robina Li. BSD 3-Clause License All Rights Reserved.
+# @file : solution.py
+# @desc : Solution for Leetcode
+# Reference : https://leetcode.com/problems/find-minimum-in-rotated-sorted-array-ii/
+
+from typing import List
+
+class Solution(object):
+    def findMin(self, nums: List[int]) -> int:
+        left, right = 0, len(nums) - 1
+        while left < right:
+            pivot = (left + right) >> 1
+            if nums[pivot] < nums[right]:
+                right = pivot
+            elif nums[pivot] > nums[right]:
+                left = pivot + 1
+            else:
+                right -= 1
+        return nums[left]
+ 
+if __name__ == '__main__':
+    solution = Solution()
+    rtn = solution.findMin([1,3,5])
+    print(1 == rtn)
+    rtn = solution.findMin([2,2,2,0,1])
+    print(0 == rtn)