[JEONGBEOMKO] WEEK 02 Solutions

3sum/JEONGBEOMKO.java

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+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+    /*
+    time complexity: O(n^2)
+    space complexity: O(1)
+     */
+        Arrays.sort(nums);
+        List<List<Integer>> result = new ArrayList<>();
+
+        for (int i = 0; i < nums.length - 2; i++) {
+            if (i > 0 && nums[i] == nums[i - 1]) continue;
+
+            int left = i + 1, right = nums.length - 1;
+
+            while (left < right) {
+                int sum = nums[i] + nums[left] + nums[right];
+
+                if (sum < 0) {
+                    left++;
+                } else if (sum > 0) {
+                    right--;
+                } else {
+                    result.add(Arrays.asList(nums[i], nums[left], nums[right]));
+                    while (left < right && nums[left] == nums[left + 1]) left++;
+                    while (left < right && nums[right] == nums[right - 1]) right--;
+                    left++; right--;
+                }
+            }
+        }
+        return result;
+    }
+}

climbing-stairs/JEONGBEOMKO.java

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+class Solution {
+    public int climbStairs(int n) {
+
+        if (n == 1 || n == 2) {
+            return n;
+        }
+
+        int[] cases = new int[n + 1];
+        cases[1] = 1;
+        cases[2] = 2;
+        for (int i = 3; i <= n; i++) {
+            cases[i] = cases[i - 1] + cases[i - 2];
+        }
+
+        return cases[n];
+    }
+}

product-of-array-except-self/JEONGBEOMKO.java

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+/* [time-complexity]: O(n)
+   [space-complexity]: O(1)
+ */
+public class JEONGBEOMKO {
+
+    class Solution {
+        public int[] productExceptSelf(int[] nums) {
+            int n = nums.length;
+            int[] result = new int[n];
+
+            // 왼쪽 곱 계산
+            result[0] = 1;
+            for (int i = 1; i < n; i++) {
+                result[i] = result[i - 1] * nums[i - 1];
+            }
+
+            // 오른쪽 곱과 동시에 결과 계산
+            int rightProduct = 1;
+            for (int i = n - 1; i >= 0; i--) {
+                result[i] = result[i] * rightProduct;
+                rightProduct *= nums[i];
+            }
+
+            return result;
+        }
+    }
+}

valid-anagram/JEONGBEOMKO.java

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+import java.util.HashMap;
+import java.util.Map;
+
+public class Solution {
+    /*
+    time complexity: O(n)
+    space complexity: O(1)
+     */
+    public boolean isAnagram(String s, String t) {
+        if (s.length() != t.length()) return false;
+
+        Map<Character, Integer> sFrequency = new HashMap<>();
+
+        for (int i = 0; i < s.length(); i++) {
+            sFrequency.put(s.charAt(i), sFrequency.getOrDefault(s.charAt(i), 0) + 1);
+        }
+
+        for (int i = 0; i < t.length(); i++) {
+            if (sFrequency.getOrDefault(t.charAt(i), 0) != 0) {
+                sFrequency.put(t.charAt(i), sFrequency.get(t.charAt(i)) - 1);
+            }
+        }
+
+        for (int count : sFrequency.values()) {
+            if (count != 0) return false;
+        }
+
+        return true;
+    }
+}

validate-binary-search-tree/JEONGBEOMKO.java

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+/**
+ * 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;
+ *     }
+ * }
+ */
+/**
+ * time complexity: O(n)
+ * space complexity: O(h)
+ */
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return isValidRange(root, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+
+    private boolean isValidRange(TreeNode root, long min, long max) {
+        if (root == null) return true;
+
+        if (root.val <= min || root.val >= max) return false;
+
+        return isValidRange(root.left, min, root.val)
+                && isValidRange(root.right, root.val, max);
+    }
+
+}