[박종훈] 5주차 답안 제출

3sum/dev-jonghoonpark.md

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+- 문제 : https://leetcode.com/problems/3sum/
+- time complexity : O(n^2)
+- space complexity : O(1) (결과값을 고려하지 않은 알고리즘 자체의 공간 복잡도)
+- 블로그 링크 : https://algorithm.jonghoonpark.com/2024/05/07/leetcode-15
+
+```java
+public List<List<Integer>> threeSum(int[] nums) {
+    Arrays.sort(nums);
+
+    List<List<Integer>> result = new ArrayList<>();
+
+    int lastOne = Integer.MIN_VALUE;
+    for (int i = 0; i < nums.length - 1; i++) {
+        int num = nums[i];
+        if (lastOne == num) {
+            continue;
+        }
+
+        twoSum(result, nums, i);
+        lastOne = num;
+    }
+
+    return result;
+}
+
+public void twoSum(List<List<Integer>> result, int[] nums, int targetIndex) {
+    int target = -nums[targetIndex];
+
+    int i = targetIndex + 1;
+    int j = nums.length - 1;
+    while (i < j) {
+        int twoSum = nums[i] + nums[j];
+
+        if (target > twoSum) {
+            i++;
+        }
+
+        if (target < twoSum) {
+            j--;
+        }
+
+        if (target == twoSum) {
+            result.add(List.of(-target, nums[i], nums[j]));
+            int current = nums[i];
+            while (i < nums.length - 2 && current == nums[i + 1]) {
+                i++;
+            }
+            i++;
+        }
+    }
+}
+```

encode-and-decode-strings/dev-jonghoonpark.md

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+- 문제
+  - 유료 : https://leetcode.com/problems/encode-and-decode-strings/
+  - 무료 : https://www.lintcode.com/problem/659/
+- time complexity : O(n)
+- space complexity : O(n \* m), m은 각 문자열 길이의 평균
+- 블로그 링크 : https://algorithm.jonghoonpark.com/2024/05/29/leetcode-271
+
+```java
+public String encode(List<String> strs) {
+    StringBuilder sb = new StringBuilder();
+    for (String str : strs) {
+        sb.append(str.replace("%", "%25").replace(",", "%2C")).append(",");
+    }
+    return sb.length() > 0 ? sb.toString() : "";
+}
+
+public List<String> decode(String str) {
+    List<String> decodedList = new ArrayList<>();
+    if (str.length() > 0) {
+        int commaIndex = str.indexOf(",");
+        while (commaIndex > -1) {
+            decodedList.add(str.substring(0, commaIndex).replace("%2C", ",").replace("%25", "%"));
+            str = str.substring(commaIndex + 1);
+            commaIndex = str.indexOf(",");
+        }
+    }
+    return decodedList;
+}
+```

longest-consecutive-sequence/dev-jonghoonpark.md

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+- 문제 : https://leetcode.com/problems/longest-consecutive-sequence/
+- time complexity : O(n)
+- space complexity : O(n)
+- 블로그 링크 : https://algorithm.jonghoonpark.com/2024/05/28/leetcode-128
+
+```java
+public int longestConsecutive(int[] nums) {
+    Set<Integer> set = new HashSet<>();
+
+    for(int num : nums) {
+        set.add(num);
+    }
+
+    int max = 0;
+    for(int num : set) {
+        if (!set.contains(num - 1)) {
+            int current = 1;
+            while (set.contains(num + 1)) {
+                current++;
+                num++;
+            }
+            max = Math.max(current, max);
+        }
+    }
+
+    return max;
+}
+```

product-of-array-except-self/dev-jonghoonpark.md

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+- 문제 : https://leetcode.com/problems/product-of-array-except-self/
+- time complexity : O(n)
+- space complexity : O(n)
+- 블로그 링크 : https://algorithm.jonghoonpark.com/2024/05/08/leetcode-238
+
+## case 나눠서 풀기
+
+```java
+public int[] productExceptSelf(int[] nums) {
+    int[] products = new int[nums.length];
+    int result = 1;
+    int zeroCount = 0;
+
+    int p = 0;
+    while (p < nums.length) {
+        if (nums[p] != 0) {
+            result *= nums[p];
+        } else {
+            zeroCount++;
+            if (zeroCount >= 2) {
+                Arrays.fill(products, 0);
+                return products;
+            }
+        }
+        p++;
+    }
+
+    if (zeroCount == 1) {
+        p = 0;
+        while (p < nums.length) {
+            if (nums[p] == 0) {
+                products[p] = result;
+            }
+            p++;
+        }
+    } else {
+        p = 0;
+        while (p < nums.length) {
+            products[p] = result / nums[p];
+            p++;
+        }
+    }
+
+    return products;
+}
+```
+
+## 재밋는 방법으로 풀기 (prefixProd)
+
+```java
+public int[] productExceptSelf(int[] nums) {
+    int[] result = new int[nums.length];
+
+    int[] prefixProd = new int[nums.length];
+    int[] suffixProd = new int[nums.length];
+
+    prefixProd[0] = nums[0];
+    for (int i = 1; i < nums.length; i++) {
+        prefixProd[i] = prefixProd[i-1] * nums[i];
+    }
+
+    suffixProd[nums.length - 1] = nums[nums.length - 1];
+    for (int i = nums.length - 2; i > -1; i--) {
+        suffixProd[i] = suffixProd[i + 1] * nums[i];
+    }
+
+    result[0] = suffixProd[1];
+    result[nums.length - 1] = prefixProd[nums.length - 2];
+    for (int i = 1; i < nums.length - 1; i++) {
+        result[i] = prefixProd[i - 1] * suffixProd[i + 1];
+    }
+
+    return result;
+}
+```

top-k-frequent-elements/dev-jonghoonpark.md

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+- 문제 : https://leetcode.com/problems/top-k-frequent-elements/
+- time complexity : O(nlogn)
+- space complexity : O(n)
+- 블로그 링크 : https://algorithm.jonghoonpark.com/2024/04/18/leetcode-347
+
+```java
+public int[] topKFrequent(int[] nums, int k) {
+    Map<Integer, Integer> counter = new HashMap<>();
+
+    Arrays.stream(nums)
+            .forEach(num -> {
+                counter.put(num, counter.getOrDefault(num, 0) + 1);
+            });
+
+    return Arrays.copyOfRange(counter.entrySet().stream()
+            .sorted(Map.Entry.<Integer, Integer>comparingByValue().reversed())
+            .mapToInt(Map.Entry::getKey)
+            .toArray(), 0, k);
+}
+```
+
+## tc, sc 관련 그렇게 생각한 이유
+
+빈도를 기준으로 map을 생성하기 때문에 n 보다 적은 경우가 대다수 이겠지만, 최악의 경우 n개의 map entry가 생성될 수 잇음.