[geegong] WEEK 01 Solutions

contains-duplicate/Geegong.java

@@ -9,17 +9,26 @@ public class Geegong {
      * time complexity : O(n)
      * space complexity : o(n)
      * @param nums
-     * @return
+     * @return boolean
      */
     public boolean containsDuplicate(int[] nums) {
+
+
+
         HashSet<Integer> uniques = new HashSet<>();
 
         for (int num : nums) {
-            if (uniques.contains(num)) {
+
+            // 명확하게 hashSet에 값이 있는지 체크하는 메소드로 확인이 가능하지만
+//            if (uniques.contains(num)) {
+//                return true;
+//            }
+//            uniques.add(num);
+
+            // hashSet 의 Add 는 이미 값이 있다면 FALSE를 리턴하기에 아래처럼도 동작 가능 (더 빠른 결과확인)
+            if (!uniques.add(num)) {
                 return true;
             }
-
-            uniques.add(num);
         }
 
         return false;

house-robber/Geegong.java

@@ -1,4 +1,52 @@
+import java.util.HashMap;
+import java.util.Map;
+
 public class Geegong {
-    // 이 문제는 시간이 남을때 풀 예정 😅
+
+    /**
+     * top-down + memoization 방식으로 풀이
+     * memoization (memo 변수) 없이 풀이하면 Time Limit Exceeded 발생
+     * time complexity : O(N) -> memo 가 있어서 이미 연산이 된건 패스함
+     * space complexity : O(N) -> index 만큼의 연산 결과가 있음
+     * @param nums
+     * @return
+     */
+    public int rob(int[] nums) {
+        // memoization 하지 않으면 Time Limit Exceeded.
+        Map<Integer, Integer> memo = new HashMap<>();
+
+        int maxAmount = 0;
+        for (int idx=0; idx<nums.length; idx++) {
+            int currAmount = Math.max(
+                    nums[idx] + rob(nums, idx+2, memo), rob(nums, idx+1, memo));
+            maxAmount = Math.max(currAmount, maxAmount);
+        }
+
+        return maxAmount;
+    }
+
+
+    public int rob(int[] origin, int currIdx, Map<Integer, Integer> memo) {
+        if (currIdx == origin.length - 1) {
+            return origin[currIdx];
+        } else if (currIdx >= origin.length) { // when out of bounds
+            return 0;
+        }
+
+        if (memo.containsKey(currIdx)) {
+            return memo.get(currIdx);
+        }
+
+        int currentVal = origin[currIdx];
+
+        int maxAmount = Math.max(
+                currentVal + rob(origin, currIdx + 2, memo), rob(origin, currIdx+1, memo));
+
+        memo.put(currIdx, maxAmount);
+
+        return maxAmount;
+    }
+
+
 }
 

longest-consecutive-sequence/Geegong.java

@@ -1,3 +1,4 @@
+import java.util.Arrays;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
@@ -7,42 +8,45 @@
  */
 public class Geegong {
 
+    /**
+     * Time complexity : O(N) + O(N long N) + O(N)
+     *    - o(N) : 한번 순회해서 set
+     *    - O(N log N) : sorting
+     *    - O (N) : sorting된걸 한번 더 순회
+     * Space complexity : O(N) -> hash set
+     * @param nums
+     * @return int
+     */
     public int longestConsecutive(int[] nums) {
-        HashSet<Integer> setOfNums = new HashSet<>();
-        // key : startIndex , value : length
-        Map<Integer, Integer> lengthMap = new HashMap<>();
-
-        // sort..? 를 해야될까 싶음..
-
-        // initialize
-        for (int num : nums) {
-            setOfNums.add(num);
+        if (nums.length == 0) {
+            return 0;
         }
 
-        Integer longest = 0;
-
-        for (Integer num : setOfNums) {
-            int length = iterate(setOfNums, num, 0, lengthMap);
-            longest = Math.max(longest, length);
+        // hashSet
+        HashSet<Integer> hashSet = new HashSet<>();
+        for (int num : nums) {
+            hashSet.add(num);
         }
 
-        return longest;
-    }
+        int[] sortedNums = hashSet.stream().mapToInt(val -> val).sorted().toArray();
 
-    public Integer iterate(HashSet<Integer> hashSet, int currIndex, int currLength, Map<Integer, Integer> lengthMap) {
-        if (lengthMap.containsKey(currIndex)) {
-            return lengthMap.get(currIndex);
-        }
+        int maxLength = 1;
+        int currentLength = 1;
+        int prev = sortedNums[0];
+        for(int index=1; index<sortedNums.length; index++) {
+            int current = sortedNums[index];
+            if (current == prev + 1) {
+                currentLength++;
 
-        if (hashSet.contains(currIndex)) {
-            currLength++;
-            return iterate(hashSet, currIndex+1, currLength, lengthMap);
+            } else {
+                maxLength = Math.max(currentLength, maxLength);
+                currentLength = 1;
+            }
 
-        } else {
-            lengthMap.put(currIndex, currLength);
-            return currLength;
+            prev = sortedNums[index];
         }
 
+        return Math.max(currentLength, maxLength);
     }
 
 }

top-k-frequent-elements/Geegong.java

@@ -1,75 +1,128 @@
-import java.util.HashMap;
-import java.util.HashSet;
-import java.util.Map;
+import java.util.*;
 
-public class Geegong {
 
 
-    public int[] topKFrequent(int[] nums, int k) {
-        int[] result = new int[k];
+public class Geegong {
 
-        // key : num element in nums / value : frequency of num elements
-        Map<Integer, Integer> numMap = new HashMap<>();
 
-        // key : frequency of num elements / value : HashSet<Integer> num elements
-        Map<Integer, HashSet<Integer>> frequencyMap = new HashMap<>();
+    /**
+     * Map 으로 빈도수를 key , 빈도수에 해당되는 num 들을 list 로 저장
+     * key (빈도수) 를 sorting
+     * k 만큼 골라낸다
+     * Time Complexity : O(N) + O(N logN) + O(N)
+     *    - O(N) : nums 만큼 iterate
+     *    - O(N log N) : sorting
+     *    - O(N) : frequency 그룹핑된 그룹 갯수만큼 iterate
+     * @param nums
+     * @param k
+     * @return int[]
+     */
+    public int[] topKFrequent(int[] nums, int k) {
 
-        // most frequent numbers
-        int maxCount = 0;
+        // key : frequency , value : list of nums
+        Map<Integer, List<Integer>> map = new HashMap<>();
+        Arrays.sort(nums);
+        int current = nums[0];
+        int count = 1;
 
-        // initialize numMap
-        for (int num : nums) {
-            if (numMap.containsKey(num)) {
-                Integer alreadyCounted = numMap.get(num);
-                numMap.put(num, alreadyCounted + 1);
+        for (int i = 1; i < nums.length; i++) {
+            if (nums[i] == current) {
+                count++;
             } else {
-                numMap.put(num, 1);
+                map.computeIfAbsent(count, el -> new ArrayList<>()).add(current);
+                current = nums[i];
+                count = 1;
             }
         }
 
+        // Add last group
+        map.computeIfAbsent(count, el -> new ArrayList<>()).add(current);
 
-        //numHashSetMap
-        for (int num : numMap.keySet()) {
-            int frequencyOfNum = numMap.get(num);
-            maxCount = Math.max(maxCount, frequencyOfNum);
-
-            if (frequencyMap.containsKey(frequencyOfNum)) {
-                HashSet<Integer> alreadySet = frequencyMap.get(frequencyOfNum);
-                alreadySet.add(num);
-
-                frequencyMap.put(frequencyOfNum, alreadySet);
-
-            } else {
-                HashSet<Integer> newHashSet = new HashSet<>();
-                newHashSet.add(num);
-
-                frequencyMap.put(frequencyOfNum, newHashSet);
-            }
-        }
+        List<Integer> sortedFrequency = map.keySet().stream().sorted(Comparator.reverseOrder()).toList();
+        List<Integer> result = new ArrayList<>();
+        for (int index=0; index<sortedFrequency.size(); index++ ) {
+            int mostFrequency = sortedFrequency.get(index);
+            List<Integer> numsOfFreq = map.get(mostFrequency);
 
+            for(int innerIndex = 0; innerIndex<numsOfFreq.size(); innerIndex++) {
+                if (result.size() == k) {
+                    return result.stream().mapToInt(Integer::intValue).toArray();
+                }
 
-        // maxCount 부터 decreasing
-        int resultIndex=0;
-        for(int frequency=maxCount; frequency>=0; frequency--) {
-            if (resultIndex >= result.length) {
-                return result;
+                result.add(numsOfFreq.get(innerIndex));
             }
 
-            if (frequencyMap.containsKey(frequency)) {
-                HashSet<Integer> numElements = frequencyMap.get(frequency);
-
-                for (int numElement : numElements) {
-                    result[resultIndex] = numElement;
-                    resultIndex++;
-
-
-                    if (resultIndex >= result.length) {
-                        return result;
-                    }
-                }
-            }
         }
 
-        return result;
+        return result.stream().mapToInt(Integer::intValue).toArray();
+
+// 아래 문제풀이는 예전 기수에 풀었던 방법으로 Map 으로 빈도수와 num을 관리하는 값을 가지긴 하나
+// sorting은 하지 않고 maxNumOfFrequency를 구하여 순차적으로 작은 값들을 꺼내서 k만큼 리턴한다
+//        int[] result = new int[k];
+//
+//        // key : num element in nums / value : frequency of num elements
+//        Map<Integer, Integer> numMap = new HashMap<>();
+//
+//        // key : frequency of num elements / value : HashSet<Integer> num elements
+//        Map<Integer, HashSet<Integer>> frequencyMap = new HashMap<>();
+//
+//        // most frequent numbers
+//        int maxCount = 0;
+//
+//        // initialize numMap
+//        for (int num : nums) {
+//            if (numMap.containsKey(num)) {
+//                Integer alreadyCounted = numMap.get(num);
+//                numMap.put(num, alreadyCounted + 1);
+//            } else {
+//                numMap.put(num, 1);
+//            }
+//        }
+//
+//
+//        //numHashSetMap
+//        for (int num : numMap.keySet()) {
+//            int frequencyOfNum = numMap.get(num);
+//            maxCount = Math.max(maxCount, frequencyOfNum);
+//
+//            if (frequencyMap.containsKey(frequencyOfNum)) {
+//                HashSet<Integer> alreadySet = frequencyMap.get(frequencyOfNum);
+//                alreadySet.add(num);
+//
+//                frequencyMap.put(frequencyOfNum, alreadySet);
+//
+//            } else {
+//                HashSet<Integer> newHashSet = new HashSet<>();
+//                newHashSet.add(num);
+//
+//                frequencyMap.put(frequencyOfNum, newHashSet);
+//            }
+//        }
+//
+//
+//        // maxCount 부터 decreasing
+//        int resultIndex=0;
+//        for(int frequency=maxCount; frequency>=0; frequency--) {
+//            if (resultIndex >= result.length) {
+//                return result;
+//            }
+//
+//            if (frequencyMap.containsKey(frequency)) {
+//                HashSet<Integer> numElements = frequencyMap.get(frequency);
+//
+//                for (int numElement : numElements) {
+//                    result[resultIndex] = numElement;
+//                    resultIndex++;
+//
+//
+//                    if (resultIndex >= result.length) {
+//                        return result;
+//                    }
+//                }
+//            }
+//        }
+//
+//        return result;
+    }
 }
 

two-sum/Geegong.java

@@ -8,26 +8,22 @@ public class Geegong {
      * space complexity : O(n)
      * @param nums
      * @param target
-     * @return
+     * @return int[]
      */
     public int[] twoSum(int[] nums, int target) {
-        Map<Integer, Integer> map = new HashMap<>();
         int[] result = new int[2];
+        // key : value, value = index
+        Map<Integer, Integer> maps = new HashMap<Integer, Integer>();
 
-        // if target = -9 / num = 1 , num = -10
-        for (int index=0; index<nums.length; index++) {
-            map.put(nums[index], index);
-        }
-
-        for (int index=0; index<nums.length; index++) {
-            int difference = target - nums[index];
-
-            if (map.containsKey(difference)
-                    && map.get(difference) != index) {
-                result[0] = index;
-                result[1] = map.get(difference);
+        for(int index=0; index<nums.length; index++) {
+            int findOne = target - nums[index];
+            if (maps.containsKey(findOne)) {
+                result[0] = maps.get(findOne);
+                result[1] = index;
                 return result;
             }
+
+            maps.put(nums[index], index);
         }
 
         return result;