[jinhyungrhee] WEEK 01 solutions

contains-duplicate/jinhyungrhee.java

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+import java.util.*;
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        /**
+         avg : O(NlogN)
+         worst : O(N^2)
+         */
+        Arrays.sort(nums);
+
+        for (int i = 1; i < nums.length; i++) {
+            if (nums[i-1] == nums[i]) return true;
+        }
+
+        return false;
+    }
+}

house-robber/jinhyungrhee.java

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+import java.util.*;
+class Solution {
+
+    // dfs with memoization => O(N)
+    public int rob(int[] nums) {
+        Map<Integer,Integer> memo = new HashMap<>();
+        return dfs(0, nums, memo);
+    }
+
+    public int dfs(int start, int[] nums, Map<Integer, Integer> memo) {
+        if (memo.containsKey(start)) return memo.get(start);
+        if (start >= nums.length) {
+            memo.put(start, 0);
+        } else {
+            memo.put(start, Math.max(
+                    nums[start] + dfs(start + 2, nums, memo),
+                    dfs(start + 1, nums, memo))
+            );
+        }
+        return memo.get(start);
+    }
+}

longest-consecutive-sequence/jinhyungrhee.java

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+import java.util.*;
+
+class Solution {
+    public int longestConsecutive(int[] nums) {
+        Set<Integer> table = new HashSet<>();
+        int longest = 0;
+
+        for(int n : nums) {
+            table.add(n);
+        }
+
+        // O(N)
+        for (int num : table) {
+            if (!table.contains(num - 1)) {
+                int currentNum = num;
+                int currentStreak = 1;
+
+                while (table.contains(currentNum + 1)) {
+                    currentNum++;
+                    currentStreak++;
+                }
+                longest = Math.max(longest, currentStreak);
+            }
+        }
+
+        /** TIME OUT 발생!
+         *
+        for (int n : nums) {
+            if (table.contains(n - 1)) continue;
+            int len = 1;
+            while (table.contains(n + len)){
+                len++;
+            }
+            longest = Math.max(len, longest);
+        }
+         */
+
+        return longest;
+    }
+}

top-k-frequent-elements/jinhyungrhee.java

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+import java.util.*;
+import java.util.stream.*;
+
+class Solution {
+    public int[] topKFrequent(int[] nums, int k) {
+
+        Map<Integer, Integer> map = new HashMap<>();
+        for(int n : nums) {
+            if(!map.containsKey(n)) {
+                map.put(n, 1);
+            } else {
+                map.put(n, map.get(n) + 1);
+            }
+        }
+
+        Map<Integer, Integer> sortedMap = map.entrySet().stream()
+                .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
+                .collect(Collectors.toMap(
+                        Map.Entry::getKey,
+                        Map.Entry::getValue,
+                        (oldVal, newVal) -> oldVal,
+                        LinkedHashMap::new
+                ));
+
+        List<Integer> list = sortedMap.keySet().stream().limit(k).toList();
+        return list.stream().mapToInt(Integer::intValue).toArray();
+    }
+}
+

two-sum/jinhyungrhee.java

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+class Solution {
+
+    // Method1. brute-force => O(N^2)
+    public int[] twoSum(int[] nums, int target) {
+        int size = nums.length;
+        int[] answer = new int[2];
+        for (int i = 0; i < size; i++) {
+            for (int j = i + 1; j < size; j++) {
+                if (nums[i] + nums[j] == target) {
+                    answer[0] = i;
+                    answer[1] = j;
+                }
+            }
+        }
+        return answer;
+    }
+}