[Yiseull] WEEK 02 solutions

3sum/Yiseull.py

@@ -0,0 +1,27 @@
+class Solution:
+    def threeSum(self, nums: list[int]) -> list[list[int]]:
+        answer = set()
+
+        nums = sorted(nums)
+
+        n = len(nums)
+        for i in range(n - 2):
+            if nums[i] > 0:
+                break
+
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            left, right = i + 1, n - 1
+            while left < right:
+                threeSum = nums[i] + nums[left] + nums[right]
+                if threeSum < 0:
+                    left += 1
+                elif threeSum == 0:
+                    answer.add((nums[i], nums[left], nums[right]))
+                    left += 1
+                    right -= 1
+                else:
+                    right -= 1
+
+        return list(answer)

climbing-stairs/Yiseull.py

@@ -0,0 +1,11 @@
+class Solution:
+    def climbStairs(self, n: int) -> int:
+        if n == 1: return 1
+
+        dp = [0 for _ in range(n + 1)]
+        dp[0], dp[1] = 1, 1
+
+        for i in range(2, n + 1):
+            dp[i] = dp[i - 1] + dp[i - 2]
+
+        return dp[n]

product-of-array-except-self/Yiseull.py

@@ -0,0 +1,15 @@
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        n = len(nums)
+        answer = [1]
+
+        # answer[i] -> nums[i] 왼쪽 값들의 곱
+        for i in range(1, n):
+            answer.append(answer[i - 1] * nums[i - 1])
+
+        tmp = 1
+        for i in range(n - 1, -1, -1):
+            answer[i] *= tmp
+            tmp *= nums[i]
+
+        return answer

valid-anagram/Yiseull.py

@@ -0,0 +1,5 @@
+from collections import Counter
+
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        return Counter(s) == Counter(t)

validate-binary-search-tree/Yiseull.java

@@ -0,0 +1,27 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return validate(root, null, null);
+    }
+
+    private boolean validate(TreeNode node, Integer low, Integer high) {
+        if (node == null) return true;
+        if (low != null && low >= node.val) return false;
+        if (high != null && high <= node.val) return false;
+        return validate(node.left, low, node.val) && validate(node.right, node.val, high);
+    }
+}