[minji-go] week 01 solutions

contains-duplicate/minji-go.java

@@ -1,24 +1,15 @@
-/*
-    Problem: https://leetcode.com/problems/contains-duplicate/
-    Description: return true if any value appears at least twice in the array
-    Concept: Array, Hash Table, Sorting
-    Time Complexity: O(n), Runtime: 10ms
-    Space Complexity: O(n), Memory: 58.6MB
-*/
-import java.util.HashSet;
-import java.util.Set;
+/**
+ <a href="https://leetcode.com/problems/contains-duplicate/">week01-1.contains-duplicate</a>
+ <li> Description: return true if any value appears at least twice in the array  </li>
+ <li> Concept: Array, Hash Table, Sorting    </li>
+ <li> Time Complexity: O(n), Runtime: 11ms   </li>
+ <li> Space Complexity: O(n), Memory: 59.27MB </li>
+ */
 
 class Solution {
+    Set<Integer> set = new HashSet<>();
+
     public boolean containsDuplicate(int[] nums) {
-        Set<Integer> count = new HashSet<>();
-        boolean answer = false;
-        for(int num : nums){
-            if(count.contains(num)) {
-                answer = true;
-                break;
-            }
-            count.add(num);
-        }
-        return answer;
+        return !Arrays.stream(nums).allMatch(set::add);
     }
 }

house-robber/minji-go.java

@@ -1,19 +1,26 @@
-/*
-    Problem: https://leetcode.com/problems/house-robber/
-    Description: the maximum amount of money you can rob if you cannot rob two adjacent houses
-    Concept: Array, Dynamic Programming
-    Time Complexity: O(n), Runtime: 0ms
-    Space Complexity: O(1), Memory: 41.42MB
-*/
+/**
+ * <a href="https://leetcode.com/problems/house-robber/">week01-5.house-robber</a>
+ * <li> Description: the maximum amount of money you can rob if you cannot rob two adjacent houses  </li>
+ * <li> Concept: Array, Dynamic Programming     </li>
+ * <li> Time Complexity: O(n), Runtime: 0ms     </li>
+ * <li> Space Complexity: O(1), Memory: 41.1MB  </li>
+ */
+
 class Solution {
     public int rob(int[] nums) {
-        int sum1 = nums[0];
-        int sum2 = nums.length>1? Math.max(nums[0], nums[1]) : nums[0];
-        for(int i=2; i<nums.length; i++){
-            int sum3=Math.max(nums[i]+sum1,sum2);
-            sum1=sum2;
-            sum2=sum3;
+        if(nums.length==1) {
+            return nums[0];
         }
-        return sum2;
+
+        List<Integer> money = new ArrayList<>();
+        money.add(nums[0]);
+        money.add(Math.max(nums[0], nums[1]));
+
+        for(int i=2; i<nums.length; i++) {
+            money.set(0, Math.max(money.get(0)+nums[i], money.get(1)));
+            Collections.swap(money, 0 , 1);
+        }
+
+        return money.get(1);
     }
 }

longest-consecutive-sequence/minji-go.java

@@ -1,32 +1,29 @@
-/*
-    Problem: https://leetcode.com/problems/longest-consecutive-sequence/
-    Description: return the length of the longest consecutive elements sequence
-    Concept: Array, Hash Table, Union Find
-    Time Complexity: O(n), Runtime: 1141ms
-    Space Complexity: O(n), Memory: 66.74MB
-*/
-import java.util.HashSet;
-import java.util.Set;
+/**
+ * <a href="https://leetcode.com/problems/longest-consecutive-sequence/">week01-4.longest-consecutive-sequence</a>
+ * <li> Description: return the length of the longest consecutive elements sequence </li>
+ * <li> Concept: Array, Hash Table, Union Find  </li>
+ * <li> Time Complexity: O(n), Runtime: 60ms  </li>
+ * <li> Space Complexity: O(n), Memory: 55.7MB </li>
+ */
 
 class Solution {
+    private Set<Integer> set;
+
     public int longestConsecutive(int[] nums) {
-        Set<Integer> set = new HashSet<>();
-        for(int num: nums) {
-            set.add(num);
-        }
+        set = Arrays.stream(nums)
+                .boxed()
+                .collect(Collectors.toSet());
 
-        int answer = 0;
-        for(int num: nums){
-            if(set.contains(num-1)){ //contains(): O(1)
-                continue;
-            }
-            int length = 1;
-            while (set.contains(num+length)){
-                length++;
-            }
-            answer = Math.max(answer, length);
-        }
+        return set.stream()
+                .filter(num -> !set.contains(num-1))
+                .map(this::calculateLength)
+                .max(Integer::compareTo)
+                .orElse(0);
+    }
 
-        return answer;
+    public int calculateLength(int num) {
+        return (int) IntStream.iterate(num, n->n+1)
+                .takeWhile(set::contains)
+                .count();
     }
 }

top-k-frequent-elements/minji-go.java

@@ -1,29 +1,23 @@
-/*
-    Problem: https://leetcode.com/problems/top-k-frequent-elements/
-    Description: return the k most frequent elements
-    Concept: Array, Hash Table, Divide and Conquer, Sorting, Heap (Priority Queue), Bucket Sort, Counting, Quickselect
-    Time Complexity: O(n log k), Runtime: 15ms
-    Space Complexity: O(n), Memory: 48.64MB
-*/
-import java.util.*;
+/**
+ * <a href="https://leetcode.com/problems/top-k-frequent-elements/">week01-3.top-k-frequent-elements</a>
+ * <li> Description: return the k most frequent elements   </li>
+ * <li> Concept: Array, Hash Table, Divide and Conquer, Sorting, Heap (Priority Queue), Bucket Sort, Counting, Quickselect </li>
+ * <li> Time Complexity: O(n log k), Runtime: 15ms     </li>
+ * <li> Space Complexity: O(n), Memory: 49.4MB        </li>
+ */
 
 class Solution {
     public int[] topKFrequent(int[] nums, int k) {
-        Map<Integer, Integer> count = new HashMap<>();
-        for(int num : nums){
-            count.put(num, count.getOrDefault(num, 0)+1);
-        }
 
-        PriorityQueue<Integer> pq = new PriorityQueue<>(Comparator.comparingInt(count::get));
-        for (int num : count.keySet()) {
-            pq.offer(num);
-            if (pq.size() > k) pq.poll();
-        }
+        Map<Integer, Integer> map = Arrays.stream(nums)
+                .boxed()
+                .collect(Collectors.toMap(num -> num, num -> 1, (cnt, cnt2) -> cnt + 1));
 
-        int[] answer = new int[k];
-        for(int i=0; i<k; i++) {
-            answer[i] = pq.poll();
-        }
-        return answer;
+        return map.entrySet().stream()
+                .sorted(Map.Entry.comparingByValue(Collections.reverseOrder()))
+                .limit(k)
+                .map(Map.Entry::getKey)
+                .mapToInt(Integer::intValue)
+                .toArray();
     }
 }

two-sum/minji-go.java

@@ -1,21 +1,24 @@
-/*
-    Problem: https://leetcode.com/problems/two-sum/
-    Description: return indices of the two numbers such that they add up to target. not use the same element twice.
-    Topics: Array, Hash Table
-    Time Complexity: O(N), Runtime 2ms
-    Space Complexity: O(N), Memory 45.1MB
-*/
+/**
+ * <a href="https://leetcode.com/problems/two-sum/">week01-2.two-sum</a>
+ * <li> Description: return indices of the two numbers such that they add up to target. not use the same element twice. </li>
+ * <li> Topics: Array, Hash Table             </li>
+ * <li> Time Complexity: O(N), Runtime 2ms    </li>
+ * <li> Space Complexity: O(N), Memory 45.4MB </li>
+ */
+
 class Solution {
     public int[] twoSum(int[] nums, int target) {
-        Map<Integer, Integer> numIndex = new HashMap<>();
-        for(int secondIndex=0; secondIndex<nums.length; secondIndex++){
-            if(numIndex.containsKey(target-nums[secondIndex])){
-                int firstIndex = numIndex.get(target-nums[secondIndex]);
-                return new int[]{firstIndex, secondIndex};
-            } else {
-                numIndex.put(nums[secondIndex], secondIndex);
-            }
+        Map<Integer, Integer> seen = new HashMap<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            int num = target - nums[i];
+
+            if (seen.containsKey(num))
+                return new int[]{seen.get(num), i};
+
+            seen.put(nums[i], i);
         }
-        return new int[]{};
+
+        throw new IllegalArgumentException();
     }
 }