Merge pull request #1211 from minji-go/week02

[minji-go] week 02 solutions

Author: minji-go

3sum/minji-go.java

@@ -1,41 +1,30 @@
-/*
-    Problem: https://leetcode.com/problems/3sum/
-    Description: return all the triplets (i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0)
-    Concept: Array, Two Pointers, Sorting
-    Time Complexity: O(N²), Runtime 70ms
-    Space Complexity: O(N), Memory 51.63MB
-*/
+/**
+ * <a href="https://leetcode.com/problems/3sum/">week02-4.3sum</a>
+ * Description: return all the triplets (i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0)
+ * Concept: Array, Two Pointers, Sorting
+ * Time Complexity: O(N²), Runtime 2021ms
+ * Space Complexity: O(N), Memory 53.9MB
+ */
+
 class Solution {
     public List<List<Integer>> threeSum(int[] nums) {
-        Map<Integer, Integer> number = new HashMap<>();
-        for(int i=0; i<nums.length; i++) {
-            number.put(nums[i], number.getOrDefault(nums[i], 0)+1);
+        Map<Integer, Integer> map = new HashMap<>();
+        for(int i=0; i<nums.length; i++){
+            map.put(nums[i], i);
         }
 
-        Arrays.sort(nums);
-        Set<List<Integer>> set = new HashSet<>();
-        List<List<Integer>> triplets = new ArrayList<>();
-        List<Integer> lastTriplet = null;
-        for(int i=0; i<nums.length-1; i++) {
-            if(i>0 && nums[i]==nums[i-1]) continue;
-
-            for(int j=i+1; j<nums.length; j++){
-                if(j>i+1 && nums[j]==nums[j-1]) continue;
-
-                int target = -(nums[i]+nums[j]);
-                if(nums[j]>target) continue;
-
-                int count = number.getOrDefault(target,0);
-                if(nums[i]==target) count--;
-                if(nums[j]==target) count--;
-                if(count<=0) continue;
-
-                List<Integer> triplet = List.of(nums[i], nums[j], target);
-                if(triplet.equals(lastTriplet)) continue;
-                lastTriplet = triplet;
-                triplets.add(triplet);
+        Set<List<Integer>> triplets = new HashSet<>();
+        for(int i=0; i<nums.length-2; i++){
+            for(int j=i+1; j<nums.length-1; j++){
+                int sum = nums[i]+nums[j];
+                if(map.containsKey(-sum) && map.get(-sum) > j){
+                    List<Integer> list = Arrays.asList(nums[i],nums[j],-sum);
+                    Collections.sort(list);
+                    triplets.add(list);
+                }
             }
         }
-        return triplets;
+
+        return new ArrayList<>(triplets);
     }
 }

climbing-stairs/minji-go.java

@@ -1,21 +1,24 @@
-/*
-    Problem: https://leetcode.com/problems/climbing-stairs/
-    Description: how many distinct ways can you climb to the top, if you can either climb 1 or 2 steps
-    Concept: Dynamic Programming, Memoization, Recursion, Math, Array, Iterator, Combinatorics ...
-    Time Complexity: O(n), Runtime: 0ms
-    Space Complexity: O(1), Memory: 40.63MB
-*/
+/**
+ * <a href="https://leetcode.com/problems/climbing-stairs/">week02-2.climbing-stairs</a>
+ * <li> Description: how many distinct ways can you climb to the top, if you can either climb 1 or 2 steps  </li>
+ * <li> Concept: Dynamic Programming, Memoization, Recursion, Math, Array, Iterator, Combinatorics ...      </li>
+ * <li> Time Complexity: O(n), Runtime: 0ms     </li>
+ * <li> Space Complexity: O(1), Memory: 40.39MB </li>
+ */
+
 class Solution {
     public int climbStairs(int n) {
-        if(n==1) return 1;
-        if(n==2) return 2;
+        if (n <= 2) return n;
+
+        int prev = 1;
+        int curr = 2;
 
-        int prev=1, cur=2;
-        for(int i=3; i<=n; i++){
-            int next=prev+cur;
-            prev=cur;
-            cur=next;
+        for (int i = 3; i <= n; i++) {
+            int next = prev + curr;
+            prev = curr;
+            curr = next;
         }
-        return cur;
+
+        return curr;
     }
 }

product-of-array-except-self/minji-go.java

@@ -1,22 +1,24 @@
-/*
-    Problem: https://leetcode.com/problems/product-of-array-except-self/
-    Description: return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i].
-    Concept: Array, Prefix Sum
-    Time Complexity: O(N), Runtime 5ms
-    Space Complexity: O(N), Memory 54.6MB - O(1) except the output array
-*/
+/**
+ <a href="https://leetcode.com/problems/product-of-array-except-self/">week02-3.product-of-array-except-self</a>
+ <li> Description: return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i].</li>
+ <li> Concept: Array, Prefix Sum                                        </li>
+ <li> Time Complexity: O(N), Runtime 2ms                                </li>
+ <li> Space Complexity: O(1), Memory 55.62MB - except the output array  </li>
+ */
 class Solution {
     public int[] productExceptSelf(int[] nums) {
+
         int[] answer = new int[nums.length];
         Arrays.fill(answer, 1);
 
-        int prefixProduct = 1;
-        int suffixProduct = 1;
-        for(int i=1; i<nums.length; i++){
-            prefixProduct = prefixProduct * nums[i-1];
-            suffixProduct = suffixProduct * nums[nums.length-i];
-            answer[i] *= prefixProduct;
-            answer[nums.length-i-1] *= suffixProduct;
+        int left = 1;
+        for(int i=1; i<nums.length; i++) {
+            answer[i] *= left *= nums[i-1];
+        }
+
+        int right = 1;
+        for(int i=nums.length-2; i>=0; i--) {
+            answer[i] *= right *= nums[i+1];
         }
 
         return answer;

valid-anagram/minji-go.java

@@ -1,25 +1,23 @@
-/*
-    Problem: https://leetcode.com/problems/valid-anagram/
-    Description: return true if one string is an anagram of the other, one formed by rearranging the letters of the other
-    Concept:String, Hash Table, Sorting, Array, Counting, String Matching, Ordered Map, Ordered Set, Hash Function ...
-    Time Complexity: O(n), Runtime: 27ms
-    Space Complexity: O(n), Memory: 43.05MB
-*/
-import java.util.HashMap;
-import java.util.Map;
+/**
+ * <a href="https://leetcode.com/problems/valid-anagram/">week02-1.valid-anagram</a>
+ * <li> Description: return true if one string is an anagram of the other, one formed by rearranging the letters of the other   </li>
+ * <li> Concept:String, Hash Table, Sorting, Array, Counting, String Matching, Ordered Map, Ordered Set, Hash Function ...      </li>
+ * <li> Time Complexity: O(n), Runtime: 15ms    </li>
+ * <li> Space Complexity: O(n), Memory: 44.66MB </li>
+ */
 
 class Solution {
     public boolean isAnagram(String s, String t) {
-        if(s.length() != t.length()) return false;
+        if (s.length() != t.length()) return false;
 
-        Map<Character, Integer> charCount = new HashMap<>();
-        for(int i=0; i<s.length(); i++){
-            charCount.put(s.charAt(i), charCount.getOrDefault(s.charAt(i), 0)+1);
-            charCount.put(t.charAt(i), charCount.getOrDefault(t.charAt(i), 0)-1);
-        }
-        for(Integer count : charCount.values()){
-            if(count !=0) return false;
-        }
-        return true;
+        Map<Integer, Integer> tmap = t.chars()
+                .boxed()
+                .collect(Collectors.toMap(i -> i, i -> 1, (i1, i2) -> i1 + i2));
+
+        Map<Integer, Integer> smap = s.chars()
+                .boxed()
+                .collect(Collectors.toMap(i -> i, i -> 1, (i1, i2) -> i1 + i2));
+
+        return tmap.equals(smap);
     }
 }