[권동현] Week 1

contains-duplicate/kdh-92.java

@@ -0,0 +1,35 @@
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        /**
+         * Constraints
+         * - 1 <= nums[] <= 10^5
+         * - -10^9 <= nums[i] <= 10^9
+         *
+         * Output
+         * - true : 중복 값 존재
+         * - false : 모든 값이 유일
+         */
+
+        // 해결법 1 (HashMap 방식 - HashSet 유사)
+        // 시간복잡도: O(N), 공간 복잡도 : O(N)
+        Map<Integer, Integer> countMap = new HashMap<>();
+
+         for (int num: nums) {
+             int count = countMap.getOrDefault(num, 0) + 1;
+             if (count > 1) return true;
+             countMap.put(num, count);
+         }
+
+         return false;
+
+        // 해결법 2 (정렬)
+        // 시간 복잡도 : O(N log N), 공간 복잡도 : O(1)
+         Arrays.sort(nums);
+
+         for (int i = 0; i < nums.length - 1; i++) {
+             if (nums[i] == nums[i + 1]) return true;
+         }
+
+         return false;
+    }
+}

house-robber/kdh-92.java

@@ -0,0 +1,29 @@
+class Solution {
+    public int rob(int[] nums) {
+        // (1) dp
+        // int n = nums.length;
+
+        // if (n == 1) return nums[0];
+
+        // int[] dp = new int[n];
+
+        // dp[0] = nums[0];
+        // dp[1] = Math.max(nums[0], nums[1]);
+
+        // for (int i = 2; i < n; i++) {
+        //     dp[i] = Math.max(dp[i - 1], nums[i] + dp[i - 2]);
+        // }
+
+        // return dp[n - 1];
+
+        // (2) 인접 값 비교
+        int prev = 0, curr = 0;
+        for (int num : nums) {
+            int temp = curr;
+            curr = Math.max(num + prev, curr);
+            prev = temp;
+        }
+
+        return curr;
+    }
+}

longest-consecutive-sequence/kdh-92.java

@@ -0,0 +1,61 @@
+/**
+ * Constraints:
+ * - 0 <= nums.length <= 105
+ * - -109 <= nums[i] <= 109
+ *
+ * Output
+ * - 가장 긴 길이
+ *
+ * 풀이 특이점
+ * - 둘다 시간복잡도는 O(N)으로 생각되는데 Runtime의 결과 차이가 꽤 크게 나온 점
+ */
+
+class Solution {
+    public int longestConsecutive(int[] nums) {
+        // (1) Set & num -1
+        // 시간복잡도 : O(N)
+        // Runtime : 1267ms Beats 5.15%
+        // Memory : 63.30MB Beats 62.58%
+        // Set<Integer> uniqueNums = Arrays.stream(nums).boxed().collect(Collectors.toSet());
+        // int result = 0;
+
+        // for (int num : nums) {
+        //     if (uniqueNums.contains(num - 1)) continue;
+        //     int length = 1;
+        //     while (uniqueNums.contains(num + length)) length += 1;
+        //     result = Math.max(length, result);
+        // }
+
+        // return result;
+
+        // (2) HashMap
+        // 시간복잡도 : O(N)
+        // Runtime : 38ms Beats 46.54%
+        // Memory : 66.45MB Beats 51.28%
+        HashMap<Integer, Boolean> hm = new HashMap<>();
+
+        for(int i=0; i<nums.length; i++){
+            hm.put(nums[i], false);
+        }
+
+        for(int key : hm.keySet()){
+            if(hm.containsKey(key - 1)){
+                hm.put(key, true);
+            }
+        }
+
+        int max = 0;
+        for(int key : hm.keySet()){
+            int k =1;
+            if(hm.get(key)){
+                while(hm.containsKey(key + k)){
+                    k++;
+                }
+            }
+
+            max = Math.max(max, k);
+        }
+
+        return max;
+    }
+}

top-k-frequent-elements/kdh-92.java

@@ -0,0 +1,82 @@
+/**
+ * Constraints
+ * 1 <= nums.length <= 10^5
+ * -10^4 <= nums[i] <= 10^4
+ * k is in the range [1, the number of unique elements in the array].
+ * It is guaranteed that the answer is unique.
+ *
+ * Output
+ * - int 배열
+ */
+
+class Solution {
+    public int[] topKFrequent(int[] nums, int k) {
+        // (1) HashMap + PriorityQueue
+        // 시간복잡도 : O(N log N)
+        // Runtime : 15ms Beats 38.03%
+        // Memory : 48.93MB Beats 20.01%
+        Map<Integer, Integer> frequencyMap = new HashMap<>();
+
+        for (int num : nums) {
+            frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
+        }
+
+        PriorityQueue<Map.Entry<Integer, Integer>> minHeap = new PriorityQueue<>(Comparator.comparingInt(Map.Entry::getValue));
+
+        for (Map.Entry<Integer, Integer> entry : frequencyMap.entrySet()) {
+            minHeap.offer(entry);
+            if (minHeap.size() > k) {
+                minHeap.poll();
+            }
+        }
+
+        int[] result = new int[k];
+
+        for (int i = 0; i < k; i++) {
+            result[i] = minHeap.poll().getKey();
+        }
+
+        return result;
+
+        // (2) Stream
+        // 시간복잡도 : O(N log N)
+        // Runtime : 19ms Beats 8.16%
+        // Memory : 49.00MB Beats 20.01%
+        // Stream에 익숙해지기 위해 공부용
+        return Arrays.stream(nums)
+                .boxed()
+                .collect(Collectors.groupingBy(num -> num, Collectors.summingInt(num -> 1)))
+                .entrySet().stream()
+                .sorted((a, b) -> b.getValue() - a.getValue())
+                .limit(k)
+                .mapToInt(Map.Entry::getKey)
+                .toArray();
+
+        // (3) Array List
+        // 시간복잡도 : O(N)
+        // Runtime : 13ms Beats 75.77%
+        // Memory : 48.44MB Beats 61.68%
+        Map<Integer, Integer> frequencyMap = new HashMap<>();
+        for (int num : nums) {
+            frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
+        }
+
+        List<Integer>[] buckets = new List[nums.length + 1];
+        for (int key : frequencyMap.keySet()) {
+            int freq = frequencyMap.get(key);
+            if (buckets[freq] == null) {
+                buckets[freq] = new ArrayList<>();
+            }
+            buckets[freq].add(key);
+        }
+
+        List<Integer> result = new ArrayList<>();
+        for (int i = buckets.length - 1; i >= 0 && result.size() < k; i--) {
+            if (buckets[i] != null) {
+                result.addAll(buckets[i]);
+            }
+        }
+
+        return result.stream().mapToInt(Integer::intValue).toArray();
+    }
+}

valid-palindrome/kdh-92.java

@@ -0,0 +1,65 @@
+/**
+ * Constraints
+ * - 1 <= s.length <= 2 * 10^5
+ * - s consists only of printable ASCII characters.
+ *
+ * Output
+ * - true : 좌우 동일
+ * - false : 좌우 비동일
+ */
+
+
+class Solution {
+    public boolean isPalindrome(String s) {
+
+        // 해결법 1 (투포인터)
+        // 시간복잡도: O(N), 공간 복잡도 : O(1)
+        // 2ms & Beats 99.05%
+        int left = 0, right = s.length() - 1;
+
+        while (left < right) {
+            // (1)
+            if (!Character.isLetterOrDigit(s.charAt(left))) {
+                left++;
+            }
+            else if (!Character.isLetterOrDigit(s.charAt(right))) {
+                right--;
+            }
+            else {
+                if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) {
+                    return false;
+                }
+
+                left++;
+                right--;
+            }
+
+
+            // (2)
+            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;
+
+        // 해결법 2 (Stream API)
+        // 시간 복잡도 : O(N), 공간 복잡도 : O(N)
+        // 133ms & Beats 5.58%
+        String filtered = s.chars()
+                .filter(Character::isLetterOrDigit)
+                .mapToObj(c -> String.valueOf((char) Character.toLowerCase(c)))
+                .reduce("", String::concat);
+
+        return IntStream.range(0, filtered.length() / 2)
+                .allMatch(i -> filtered.charAt(i) == filtered.charAt(filtered.length() - 1 - i));
+    }
+}