[ackku] Week 1

contains-duplicate/imsosleepy.java

@@ -0,0 +1,15 @@
+// 중복제거를 위해 set을 적극적으로 활용해야할 듯...
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        Set<Integer> numSet = new HashSet<>();
+
+        for (int num : nums) {
+            if (numSet.contains(num)) {
+                return true;
+            }
+            numSet.add(num);
+        }
+
+        return false;
+    }
+}

house-robber/imsosleepy.java

@@ -0,0 +1,33 @@
+// 공간복잡도를 줄이는법. 배열로 관리 안하기
+class Solution {
+    public int rob(int[] nums) {
+        if (nums.length == 1) return nums[0];
+
+        int prev2 = nums[0]; // dp[i-2]
+        int prev1 = Math.max(nums[0], nums[1]); // dp[i-1]
+        for (int i = 2; i < nums.length; i++) {
+            int current = Math.max(nums[i] + prev2, prev1);
+            prev2 = prev1;
+            prev1 = current;
+        }
+        return prev1;
+    }
+}
+
+// 점화식의 최대값을 구하는 방법
+// 1. 현재 위치의 최대 값은 한칸 전 집까지만 털었던가(두칸 연속 겹치면 안된다는 룰을 지키면서)
+// 2. 두칸 전 집까지 털고 + 현재집을 털었을 때다
+class Solution {
+    public int rob(int[] nums) {
+        if (nums.length == 1) {
+            return nums[0];
+        }
+        int[] dp = new int[nums.length];
+        dp[0] = nums[0];
+        dp[1] = Math.max(nums[0], nums[1]);
+        for (int i = 2; i < nums.length; i++) {
+            dp[i] = Math.max(nums[i] + dp[i - 2], dp[i - 1]);
+        }
+        return dp[nums.length - 1];
+    }
+}

longest-consecutive-sequence/imsosleepy.java

@@ -0,0 +1,79 @@
+// 중복여부만 제거하고 포함여부로 판단 O(N)
+class Solution {
+    public int longestConsecutive(int[] nums) {
+        if (nums == null || nums.length == 0) {
+            return 0;
+        }
+
+        HashSet<Integer> set = new HashSet<>();
+        for (int num : nums) {
+            set.add(num);
+        }
+
+        int maxLength = 0;
+
+        for (int num : set) {
+            if (!set.contains(num - 1)) {
+                int currentNum = num;
+                int count = 1;
+                while (set.contains(currentNum + 1)) {
+                    currentNum++;
+                    count++;
+                }
+
+                maxLength = Math.max(maxLength, count);
+            }
+        }
+
+        return maxLength;
+    }
+}
+// 정렬이 들어가면 O(nlogn) 아래로 줄일 수 없음
+class Solution {
+    public int longestConsecutive(int[] nums) {
+        if(nums.length == 0) return 0;
+        TreeSet<Integer> set = new TreeSet<>();
+        for (int num : nums) {
+            set.add(num);
+        }
+        int max = 1;
+        int consecutiveCount = 1;
+        int prev = set.pollFirst();
+        while(!set.isEmpty()) {
+            int next = set.pollFirst();
+            if (next - prev == 1) {
+                consecutiveCount++;
+            } else {
+                max = Math.max(consecutiveCount, max);
+                consecutiveCount = 1;
+            }
+            prev = next;
+        }
+        return Math.max(max, consecutiveCount);
+    }
+}
+// 이중 변환 필요 없음
+class Solution {
+    public int longestConsecutive(int[] nums) {
+        if(nums.length == 0) return 0;
+        HashSet<Integer> set = new HashSet<>();
+        for (int num : nums) {
+            set.add(num);
+        }
+        PriorityQueue<Integer> pq = new PriorityQueue<>(set);
+        int max = 1;
+        int consecutiveCount = 1;
+        int prev = pq.poll();
+        while(!pq.isEmpty()) {
+            int next = pq.poll();
+            if (next - prev == 1) {
+                 consecutiveCount++;
+             } else {
+                max = Math.max(consecutiveCount, max);
+                consecutiveCount = 1;
+            }
+            prev = next;
+        }
+        return Math.max(max, consecutiveCount);
+    }
+}

top-k-frequent-elements/imsosleepy.java

@@ -0,0 +1,20 @@
+class Solution {
+    public int[] topKFrequent(int[] nums, int k) {
+        Map<Integer, Integer> countMap = new HashMap<>();
+        for (int num : nums) {
+            countMap.put(num, countMap.getOrDefault(num, 0) + 1);
+        }
+
+        PriorityQueue<Integer> pq = new PriorityQueue<>(
+                Comparator.comparingInt(countMap::get).reversed()
+        );
+
+        pq.addAll(countMap.keySet());
+
+        int[] result = new int[k];
+        for (int i = 0; i < k; i++) {
+            result[i] = pq.poll();
+        }
+        return result;
+    }
+}

valid-palindrome/imsosleepy.java

@@ -0,0 +1,20 @@
+// 정규표현식으로 풀기엔 재미없어보여서 Character.isLetterOrDigit을 이용함
+class Solution {
+    public boolean isPalindrome(String s) {
+        int left = 0
+        int right = s.length() - 1;
+
+        while (left < right) {
+            while (left < right && !Character.isLetterOrDigit(s.charAt(left))) left++;
+            while (left < right && !Character.isLetterOrDigit(s.charAt(right))) right--;
+
+            if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) {
+                return false;
+            }
+            left++;
+            right--;
+        }
+
+        return true;
+    }
+}