Week1 Solutions

Author: hoyeongkwak

contains-duplicate/hoyeongkwak.java

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+/*
+time complexity : O(n)
+space complexity : O(n)
+*/
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        HashSet<Integer> uniqueNums = new HashSet<Integer>();
+
+        for (int num:nums) {
+            if (!uniqueNums.add(num)) {
+                return true;
+            }
+        }
+        return false;
+    }
+}

house-robber/hoyeongkwak.java

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+/*
+time complexity : O(n)
+space complexity : O(n)
+*/
+class Solution {
+    public int rob(int[] nums) {
+        int[] dp = new int[nums.length + 1];
+        dp[0] = 0;
+        dp[1] = nums[0];
+        for (int i = 2; i < dp.length; i++) {
+            dp[i] = Math.max(dp[i - 1], dp[i - 2] + nums[i - 1]);
+        }
+        return dp[dp.length - 1];
+    }
+}

longest-consecutive-sequence/hoyeongkwak.java

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

top-k-frequent-elements/hoyeongkwak.java

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+/*
+time complexity : O(nlogn)
+space complexity : O(n)
+*/
+class Solution {
+    public int[] topKFrequent(int[] nums, int k) {
+        HashMap<Integer, Integer> numMap = new HashMap<>();
+        for (int num:nums) {
+            if (numMap.containsKey(num)) {
+                int value = numMap.get(num);
+                numMap.put(num, value + 1);
+            } else {
+                numMap.put(num, 1);
+            }
+        }
+        List<Map.Entry<Integer, Integer>> entryList = new ArrayList<>(numMap.entrySet());
+        entryList.sort((a, b) -> b.getValue() - a.getValue());
+        int[] result = new int[k];
+        for (int i = 0; i < k; i++) {
+            result[i] = entryList.get(i).getKey();
+        }
+        return result;
+    }
+}

two-sum/hoyeongkwak.java

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+/*
+time complexity : O(n)
+space complexity : O(n)
+*/
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        HashMap<Integer, Integer> sumMap = new HashMap<>();
+        for (int i = 0; i < nums.length; i++) {
+            int secondNum = target - nums[i];
+            if (sumMap.containsKey(secondNum)) {
+                return new int[] { sumMap.get(secondNum), i };
+            }
+            sumMap.put(nums[i], i);
+        }
+        return new int[] {};
+    }
+}