[njngwn] WEEK 05 solutions

best-time-to-buy-and-sell-stock/njngwn.java

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+// Time Complexity: O(n), n: prices.length
+// Space Complexity: O(1), because 3 int-variables are declared -> O(1)+O(1)+O(1)
+class Solution {
+    public int maxProfit(int[] prices) {
+        int buyPrice = prices[0];
+        int sellPrice = 0;
+        int profit = 0;  // store maxProfit
+
+        for (int i = 1; i < prices.length; ++i) {
+            buyPrice = Math.min(buyPrice, prices[i]);
+            profit = Math.max(profit, prices[i] - buyPrice);
+        }
+
+        return profit;
+    }
+}

group-anagrams/njngwn.java

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+// Time Complexity: O(n), n: strs.length
+// Space Complexity: O(m+n), m: strMap.size, n: anagramList.size
+class Solution {
+    public List<List<String>> groupAnagrams(String[] strs) {
+        HashMap<String, ArrayList<String>> strMap = new HashMap<String, ArrayList<String>>();
+
+        for (String str : strs) {
+            char[] charArr = str.toCharArray();
+            Arrays.sort(charArr);
+            String key = String.valueOf(charArr);
+
+            if (strMap.containsKey(key)) {
+                strMap.get(key).add(str);
+            } else {
+                strMap.put(key, new ArrayList<String>(Arrays.asList(str)));
+            }
+        }
+
+        return new ArrayList<>(strMap.values());
+    }
+}

implement-trie-prefix-tree/njngwn.java

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+// Time Complexity: O(n), n: word.length()
+// Space Complexity: O(n), n: word.length()
+class Trie {
+    private static class TrieNode {
+        HashMap<Character, TrieNode> trieNodeMap;
+        boolean isEnd;
+
+        private TrieNode() {
+            this.trieNodeMap = new HashMap<>();
+            this.isEnd = false;
+        }
+    }
+
+    private TrieNode rootNode;
+
+    public Trie() {
+        this.rootNode = new TrieNode();
+    }
+
+    public void insert(String word) {
+        TrieNode currentNode = this.rootNode;
+
+        for (char ch : word.toCharArray()) {
+            if (!currentNode.trieNodeMap.containsKey(ch)) {
+                currentNode.trieNodeMap.put(ch, new TrieNode());
+            }
+            currentNode = currentNode.trieNodeMap.get(ch);
+        }
+        currentNode.isEnd = true;
+    }
+
+    public boolean search(String word) {
+        TrieNode currentNode = this.rootNode;
+
+        for (char ch : word.toCharArray()) {
+            if (!currentNode.trieNodeMap.containsKey(ch)) {
+                return false;
+            }
+            currentNode = currentNode.trieNodeMap.get(ch);
+        }
+
+        return currentNode.isEnd;
+    }
+
+    public boolean startsWith(String prefix) {
+        TrieNode currentNode = this.rootNode;
+
+        for (char ch : prefix.toCharArray()) {
+            if (!currentNode.trieNodeMap.containsKey(ch)) {
+                return false;
+            }
+            currentNode = currentNode.trieNodeMap.get(ch);
+        }
+
+        return true;
+    }
+}
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * Trie obj = new Trie();
+ * obj.insert(word);
+ * boolean param_2 = obj.search(word);
+ * boolean param_3 = obj.startsWith(prefix);
+ */