[bky373] Solve week 5 problems

3sum/bky373.java

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+/**
+ * time: O(n^2)
+ * space: O(n)
+ *
+ * - time: becasue of two nested loop and inner loop having a linear time complexity.
+ * - space: because of a HashSet to store the triplets.
+ */
+class Solution {
+
+    public List<List<Integer>> threeSum(int[] nums) {
+        Arrays.sort(nums);
+        int i = 0, j, k;
+        int ni = 0, nj, nk;
+        Set<List<Integer>> res = new HashSet<>();
+        while (i < nums.length && ni <= 0) {
+            ni = nums[i];
+            j = i + 1;
+            k = nums.length - 1;
+            while (j < k) {
+                nj = nums[j];
+                nk = nums[k];
+                int sum = ni + nj + nk;
+                if (sum < 0) {
+                    j++;
+                } else if (sum > 0) {
+                    k--;
+                } else {
+                    res.add(List.of(ni, nj, nk));
+                    j++;
+                }
+            }
+            i++;
+        }
+        return res.stream()
+                  .toList();
+    }
+}

encode-and-decode-strings/bky373.java

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+/**
+ * time: O(N)
+ * space: O(N)
+ */
+public class Codec {
+
+    // Encodes a list of strings to a single string.
+    public String encode(List<String> strs) {
+        StringBuilder sb = new StringBuilder();
+        for (String str : strs) {
+            sb.append(str.length())
+              .append(':')
+              .append(str);
+        }
+        return sb.toString();
+    }
+
+    // Decodes a single string to a list of strings.
+    public List<String> decode(String s) {
+        List<String> decoded = new ArrayList<>();
+        int i = 0;
+        while (i < s.length()) {
+            int searchIndex = s.indexOf(':', i);
+            int chunkSize = Integer.parseInt(s.substring(i, searchIndex));
+            i = searchIndex + chunkSize + 1;
+            decoded.add(s.substring(searchIndex + 1, i));
+        }
+        return decoded;
+    }
+}

longest-consecutive-sequence/bky373.java

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+/**
+ * time: O(N)
+ * space: O(N)
+ */
+class Solution {
+
+    public int longestConsecutive(int[] nums) {
+        Set<Integer> numSet = new HashSet<>();
+        for (int n : nums) {
+            numSet.add(n);
+        }
+        int longest = 0;
+        for (int n : nums) {
+            if (numSet.contains(n - 1)) {
+                continue;
+            }
+            int seq = 1;
+            while (numSet.contains(n + seq)) {
+                seq++;
+            }
+            longest = Math.max(longest, seq);
+        }
+        return longest;
+    }
+}

product-of-array-except-self/bky373.java

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+/**
+ * time: O(N)
+ * space: O(N)
+ */
+class Solution {
+    public int[] productExceptSelf(int[] nums) {
+        int len = nums.length;
+        int[] res = new int[len];
+        int[] left = Arrays.copyOf(nums, len);
+        int[] right = Arrays.copyOf(nums, len);
+        for (int i = 1; i < len; i++) {
+            left[i] *= left[i - 1];
+        }
+        for (int i = len - 2; i >= 0; i--) {
+            right[i] *= right[i + 1];
+        }
+        for (int i = 1; i < len - 1; i++) {
+            res[i] = left[i - 1] * right[i + 1];
+        }
+        res[0] = right[1];
+        res[len - 1] = left[len - 2];
+        return res;
+    }
+}

top-k-frequent-elements/bky373.java

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+/**
+ * time: O(N log k)
+ * space: O(N + k)
+ *
+ * - time: N 개를 순회하며 heap 에 요소 추가, 최대 k 개만큼 요소를 추가하여 log k 만큼 시간이 곱해짐.
+ * - space: 빈도 수를 저장하기 위해 N, 힙에 요소를 저장하기 위해 K 만큼 저장 공간을 사용함.
+ */
+class Solution {
+
+    public int[] topKFrequent(int[] nums, int k) {
+        Map<Integer, Integer> freqMap = new HashMap<>();
+        for (int n : nums) {
+            freqMap.put(n, freqMap.getOrDefault(n, 0) + 1);
+        }
+        Queue<Integer> pq = new PriorityQueue<>(Comparator.comparingInt(freqMap::get));
+        for (Integer n : freqMap.keySet()) {
+            pq.add(n);
+            if (pq.size() > k) {
+                pq.poll();
+            }
+        }
+        return IntStream.range(0, k)
+                        .map(i -> pq.poll())
+                        .toArray();
+    }
+}