[samcho0608] WEEK 01 solutions

contains-duplicate/samcho0608.java

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+import java.util.HashSet;
+
+// link: https://leetcode.com/problems/contains-duplicate/description/
+// difficulty: Easy
+class Solution {
+    // Problem:
+    // * return: does any value appear more than once in the array
+    // Solution:
+    // * Time Complexity: O(N)
+    // * Space Complexity: O(N)
+    public boolean containsDuplicate(int[] nums) {
+        // Space Complexity: O(N)
+        HashSet<Integer> uniqNums = new HashSet<>();
+
+        // Time Complexity: O(N)
+        for(int num : nums) {
+            if(!uniqNums.add(num)) return true;
+        }
+
+        return false;
+    }
+}

house-robber/samcho0608.java

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+// link: https://leetcode.com/problems/house-robber/description/
+// difficulty: Medium
+class Solution {
+    // Problem:
+    // * can't rob two adj houses in the same night
+    // * return: max amount of money robbale in one night
+    // Solution:
+    // * Time Complexity: O(N)
+    // * Space Complexity: O(N)
+    public int rob(int[] nums) {
+        if(nums.length == 1) return nums[0];
+        if(nums.length == 2) return Math.max(nums[0], nums[1]);
+
+        // maxSum[i] = max sum possible with nums[i]
+        int[] maxSum = new int[nums.length];
+        maxSum[0] = nums[0];
+        maxSum[1] = nums[1];
+
+        for(int i = 2; i < nums.length; i++) {
+            if(i == 2) {
+                maxSum[i] = nums[i] + maxSum[i-2];
+                continue;
+            }
+
+            // adj houses(i-1) can't be robbed
+            // choices are:
+            // 1. i-2 th house (choosing without skipping a house)
+            // 2. i-3 th house (choosing with skipping a house)
+            //    * choosing < i-4 th houses wouldn't be optimal because it'll be missing either of 1. or 2.
+            maxSum[i] = nums[i] + Math.max(maxSum[i-2], maxSum[i-3]);
+        }
+
+        return Math.max(maxSum[nums.length-2], maxSum[nums.length-1]);
+    }
+}

longest-consecutive-sequence/samcho0608.java

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+import java.util.HashSet;
+import java.util.Set;
+
+// link: https://leetcode.com/problems/longest-consecutive-sequence/
+// difficulty: Medium
+class Solution {
+    // Problem: 
+    // * nums is unsorted
+    // * return: length of longest consecutive elements sequence
+    // * req: O(N) time
+    // Solution:
+    // * Time Complexity: O(N)
+    // * Space Complexity: O(N)
+    public int longestConsecutive(int[] nums) {
+        // Time Complexity: O(N)
+        // Space Complexity: O(N)
+        Set<Integer> uniq = new HashSet<>();
+        for(int num : nums) {
+            uniq.add(num);
+        }
+
+        // Time Complexity: O(N)
+        // * nested loop but is O(N) due to skipping non-root element
+        int maxLen = 0;
+        for(int num : uniq) {
+            // skip if num isn't the root(aka first number in the sequence)
+            if(uniq.contains(num - 1)) continue;
+
+            // count till end of consecutive sequence
+            int len = 1;
+            for(int i = 1; uniq.contains(num + i); i++) {
+                len++;
+            }
+
+            maxLen = Math.max(maxLen, len);
+        }
+
+        return maxLen;
+    }
+}

top-k-frequent-elements/samcho0608.java

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+import java.util.*;
+
+// link: https://leetcode.com/problems/top-k-frequent-elements/description/
+// difficulty: Medium
+
+// Time complexity: O(Nlogk)
+// Space complexity: O(N)
+class Solution1 {
+    // return: top k most freq elements
+    public int[] topKFrequent(int[] nums, int k) {
+        HashMap<Integer, Integer> freq = new HashMap<>();
+        // O (N)
+        for(int num : nums) {
+            freq.put(num, freq.getOrDefault(num, 0) + 1); // O(1)
+        }
+
+        // O (N log k)
+        PriorityQueue<int[]> heap = new PriorityQueue<>(Comparator.comparingInt(a -> a[1]));
+        for(int num : freq.keySet()) {
+            int f = freq.get(num); // O(1)
+            heap.add(new int[]{num, f}); // O(log k)
+
+            if(heap.size() > k) heap.poll(); // O(log k)
+        }
+
+        // O (N log k)
+        int[] result = new int[k];
+        for(int i = 0; i < k; i++) {
+            result[i] = heap.poll()[0]; // O(log k)
+        }
+
+        return result;
+    }
+}
+
+// Time complexity: O(N)
+// Space complexity: O(N)
+class Solution2 {
+    public int[] topKFrequent(int[] nums, int k) {
+        // count frequencies
+        Map<Integer, Integer> freq = new HashMap<>();
+        for (int num : nums) {
+            freq.put(num, freq.getOrDefault(num, 0) + 1);
+        }
+
+        // bucket: index = frequency, value = list of numbers
+        List<List<Integer>> buckets = new ArrayList<>(nums.length + 1);
+        for (int i = 0; i <= nums.length; i++) {
+            buckets.add(new ArrayList<>());
+        }
+
+        for (var entry : freq.entrySet()) {
+            int num = entry.getKey();
+            int count = entry.getValue();
+            buckets.get(count).add(num);
+        }
+
+        // gather top k frequent elements
+        int[] result = new int[k];
+        int idx = 0;
+        for (int i = nums.length; i >= 0 && idx < k; i--) {
+            for (int num : buckets.get(i)) {
+                result[idx++] = num;
+                if (idx == k) break;
+            }
+        }
+
+        return result;
+    }
+}

two-sum/samcho0608.java

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+import java.util.HashMap;
+import java.util.Map;
+
+// link: https://leetcode.com/problems/two-sum/description/
+// difficulty: Easy
+class Solution {
+    // Problem
+    // * exactly one solution
+    // * must use index only once
+    // * return: indices of two numbers that add up to `target`
+    // Solution:
+    // * Time Complexity: O(N)
+    // * Space Complexity: O(N)
+    public int[] twoSum(int[] nums, int target) {
+        // Space Complexity: O(N)
+        Map<Integer, Integer> indexByNum = new HashMap<>();
+
+        // Time Complexity: O(N)
+        for(int i = 0; i < nums.length; i++) {
+            int numI = nums[i];
+            indexByNum.put(numI, i);
+        }
+
+        // Time Complexity: O(N)
+        for(int i = 0; i < nums.length; i++) {
+            int numI = nums[i];
+            Integer compl = indexByNum.getOrDefault(target-numI, null);
+
+            if(compl != null && i != compl)
+                return new int[] {i, compl};
+        }
+
+        return null;
+    }
+}