[dylan-jung] WEEK 01 solutions

contains-duplicate/dylan-jung.cpp

@@ -0,0 +1,13 @@
+// TC: O(N), SC: O(N)
+
+class Solution {
+public:
+    bool containsDuplicate(vector<int>& nums) {
+        unordered_set<int> s;
+        for(auto item: nums) {
+            if(s.count(item) > 0) return true;
+            s.insert(item);
+        }
+        return false;
+    }
+};

house-robber/dylan-jung.cpp

@@ -0,0 +1,18 @@
+// TC: O(N), SC: O(1)
+class Solution {
+public:
+    int rob(vector<int>& nums) {
+        if (nums.size() == 1) return nums[0];
+
+        int prev2 = nums[0];        // dp[i-2]
+        int prev1 = max(nums[0], nums[1]); // dp[i-1]
+
+        for (int i = 2; i < nums.size(); i++) {
+            int cur = max(prev1, prev2 + nums[i]); 
+            prev2 = prev1;
+            prev1 = cur;
+        }
+
+        return prev1;
+    }
+};

longest-consecutive-sequence/dylan-jung.cpp

@@ -0,0 +1,24 @@
+// TC: O(N), SC: O(N)
+
+class Solution {
+public:
+    int longestConsecutive(vector<int>& nums) {
+        auto s = unordered_set<int>();
+        for(int it : nums) {
+            s.insert(it);
+        }
+
+        int m = 0;
+        for(int it : s) {
+            if(s.count(it-1) > 0) continue;
+
+            int cnt = 0;
+            while(s.count(it+cnt) > 0){
+                cnt++;
+            }
+            m = max(m, cnt);
+        }
+
+        return m;
+    }
+};

top-k-frequent-elements/dylan-jung.cpp

@@ -0,0 +1,31 @@
+// TC: O(N), SC: O(N)
+class Solution {
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        unordered_map<int, int> freq;
+        freq.reserve(nums.size() * 2);
+        for (int x : nums) {
+            ++freq[x];
+        }
+
+        int n = nums.size();
+        vector<vector<int>> bucket(n + 1);
+        for (auto& p : freq) {
+            int num = p.first;
+            int cnt = p.second;
+            bucket[cnt].push_back(num);
+        }
+
+        vector<int> ans;
+        ans.reserve(k);
+
+        for (int count = n; count >= 1 && ans.size() < k; --count) {
+            for (int num : bucket[count]) {
+                ans.push_back(num);
+                if (ans.size() == k) break;
+            }
+        }
+
+        return ans;
+    }
+};

two-sum/dylan-jung.cpp

@@ -0,0 +1,17 @@
+// TC: O(N), SC: O(N)
+
+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        unordered_map<int, int> m;
+        for(int i = 0; i < nums.size(); i++) {
+            m[nums[i]] = i;
+        }
+        for(int first = 0; first < nums.size(); first++) {
+            if(m.count(target - nums[first]) > 0 && first != m[target - nums[first]]) {
+                return {first, m[target - nums[first]]};
+            }
+        }
+        return {};
+    }
+};