[Leo] 8th Week solutions (first 3 Qs)

Author: leokim0922

combination-sum/Leo.py

@@ -0,0 +1,14 @@
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        dp = [[] for i in range(target + 1)]
+        dp[0] = [[]]
+
+        for num in candidates:
+            for i in range(num, target + 1):
+                for comb in dp[i - num]:
+                    dp[i].append(comb + [num])
+
+        return dp[target]
+
+        ## TC: O(outer loop * 1st inner(target size) * 2nd inner(combination #))
+        ## SC: O(target size * combination #)

construct-binary-tree-from-preorder-and-inorder-traversal/Leo.py

@@ -0,0 +1,18 @@
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        if len(inorder) == 0:
+            return None
+
+        if len(preorder) == 1:
+            return TreeNode(preorder[0])
+
+        idx = inorder.index(preorder.pop(0))
+        node = TreeNode(inorder[idx])
+
+        node.left = self.buildTree(preorder, inorder[:idx])
+        node.right = self.buildTree(preorder, inorder[idx + 1:])
+
+        return node
+
+
+        ## TC: O(n^2), SC: O(n)

kth-smallest-element-in-a-bst/Leo.py

@@ -0,0 +1,18 @@
+class Solution:
+    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
+        ans = []
+
+        def helper(node):
+            if not node: return
+            helper(node.left)
+
+            if len(ans) == k:
+                return
+
+            ans.append(node.val)
+            helper(node.right)
+
+        helper(root)
+        return ans[-1]
+
+        ## TC: O(n), SC: O(h+n) where h == heights(tree) and n == element(tree)