[std-freejia] week 01 solutions

contains-duplicate/std-freejia.java

@@ -0,0 +1,13 @@
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        Set<Integer> set = new HashSet<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            if (i > 0 && set.contains(nums[i])) {
+                return true;
+            }
+            set.add(nums[i]);
+        }
+        return false;
+    }
+}

house-robber/std-freejia.java

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+class Solution {
+    public int rob(int[] nums) {
+        int numsLen = nums.length;
+        int[] dp = new int[numsLen];
+
+        if (numsLen == 1) return nums[0];
+        dp[0] = nums[0]; 
+        dp[1] = Math.max(nums[0], nums[1]); 
+
+        for (int i = 2; i < numsLen; i++){
+            dp[i] = Math.max(dp[i-2] + nums[i], dp[i-1]);
+        }
+        return dp[numsLen-1];
+    }
+}

longest-consecutive-sequence/std-freejia.java

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+/** 시간초과 발생합니다. 해법 찾는 중입니다 */
+class Solution {
+    public int longestConsecutive(int[] nums) {
+
+        Set<Integer> uniqueNum = new HashSet<>();
+        for (int num : nums) { // 수열 내 중복 숫자 제거
+            uniqueNum.add(num);
+        }
+
+        int uniqueLen = uniqueNum.size();
+        int longest = 0;
+
+        for(int targetNum: uniqueNum){
+
+            int n = 1;
+            for(int j = targetNum + 1; j < targetNum + uniqueLen; j++) {
+                if (!uniqueNum.contains(j)) break;
+                n++;
+            }
+            longest = Math.max(n, longest);
+
+        }
+        return longest;
+    }
+}

top-k-frequent-elements/std-freejia.java

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+class Solution {
+    public int[] topKFrequent(int[] nums, int k) {
+        Map<Integer, Integer> map = new HashMap<>();
+        for (int i = 0; i < nums.length; i++) {
+            map.put(nums[i], map.getOrDefault(nums[i], 0) + 1);
+        }
+        // value 를 기준으로 정렬
+        List<Map.Entry<Integer, Integer>> entrySet = new ArrayList<>(map.entrySet());
+        entrySet.sort((a, b) -> b.getValue().compareTo(a.getValue()));
+
+        int[] result = new int[k];
+        for (int i = 0; i < k; i++) {
+
+            result[i] =  entrySet.get(i).getKey();
+        }
+        return result;
+    }
+}

two-sum/std-freejia.java

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+class Solution {
+    public static int[] twoSum(int[] nums, int target) {
+        HashMap<Integer, Integer> map = new HashMap<>();
+
+        for (int first = 0; first < nums.length; first++) {
+            int targetKey = target - nums[first];
+            if (map.containsKey(targetKey)) {
+                int second = map.get(targetKey);
+                return new int[]{second, first};
+            }
+            map.put(nums[first], first);
+        }
+
+        return null;
+    }
+}