Merge pull request #851 from forest000014/main

[forest000014] Week 5

best-time-to-buy-and-sell-stock/forest000014.java

@@ -0,0 +1,30 @@
+/*
+Time complexity: O(n)
+Space complexity: O(1)
+
+i <= j 인 두 인덱스 i, j에 대해서, prices[j] - prices[i]를 최대화해야 한다.
+
+1. i = 0부터 시작하여, 오른쪽으로 순회한다.
+2. 현재 값이 max보다 크다면, max를 갱신하고, min과의 차이를 계산한다.
+3. 현재 값이 min보다 작다면, min을 갱신하고, max 역시 같은 값으로 갱신한다. (과거로 돌아가서 팔 수는 없으므로)
+*/
+class Solution {
+    public int maxProfit(int[] prices) {
+        int min = 999999;
+        int max = 0;
+        int ans = 0;
+        for (int i = 0; i < prices.length; i++) {
+            if (prices[i] > max) {
+                max = prices[i];
+                if (max - min > ans) {
+                    ans = max - min;
+                }
+            }
+            if (prices[i] < min) {
+                min = max = prices[i];
+            }
+        }
+
+        return ans;
+    }
+}

encode-and-decode-strings/forest000014.java

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+/*
+Time Complexity: O(n)
+Space Complexity: O(1)
+
+non-ASCII 유니코드 문자를 사용함
+
+To-Do : escaping 방법 학습하기
+*/
+public class Codec {
+
+    // Encodes a list of strings to a single string.
+    public String encode(List<String> strs) {
+        StringBuilder sb = new StringBuilder();
+        for (int i = 0; i < strs.size(); i++) {
+            if (i > 0) {
+                sb.append('\u2764');
+            }
+            sb.append(strs.get(i));
+        }
+
+        return sb.toString();
+    }
+
+    // Decodes a single string to a list of strings.
+    public List<String> decode(String s) {
+        return new ArrayList<>(Arrays.asList(s.split("\u2764")));
+    }
+}
+
+// Your Codec object will be instantiated and called as such:
+// Codec codec = new Codec();
+// codec.decode(codec.encode(strs));

group-anagrams/forest000014.java

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+/*
+Time Complexity: O(n)
+Space Complexity: O(n)
+*/
+class Solution {
+    public List<List<String>> groupAnagrams(String[] strs) {
+        Map<String, List<String>> map = new HashMap<>();
+
+        for (String str : strs) {
+            int[] cnt = new int[26];
+            for (int i = 0; i < str.length(); i++) {
+                cnt[str.charAt(i) - 'a']++;
+            }
+            char[] chars = new char[str.length()];
+            int idx = 0;
+            for (int i = 0; i < 26; i++) {
+                while (cnt[i] > 0) {
+                    chars[idx++] = (char)(i + 'a');
+                    cnt[i]--;
+                }
+            }
+            String sorted = new String(chars);
+            if (!map.containsKey(sorted)) {
+                map.put(sorted, new ArrayList<>());
+            }
+            map.get(sorted).add(str);
+        }
+
+        List<List<String>> ans = new ArrayList<>();
+        for (String key : map.keySet()) {
+            ans.add(new ArrayList<>());
+            for (String str : map.get(key)) {
+                ans.get(ans.size() - 1).add(str);
+            }
+        }
+
+        return ans;
+    }
+}

implement-trie-prefix-tree/forest000014.java

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+/*
+Time Complexity (n = length of word/prefix)
+- initialization: O(1)
+- insert: O(n)
+- search: O(n)
+- startsWith: O(n)
+
+Space Complexity: O(n * c) (c = calls)
+- 길이 3인 알파벳 소문자 문자열의 가짓수 : 26^3 = 17,576
+- 길이 4인 알파벳 소문자 문자열의 가짓수 : 26^4 = 456,976
+만약 n이 3 이하였다면, 3만 번의 call 동안 trie가 포화되어 공간 복잡도가 O(26 * n) = O(n) 이었을 것.
+하지만 n이 2,000으로 충분히 크기 때문에 trie가 포화되지는 않을 것이므로, 공간 복잡도는 O(n * c).
+*/
+class Trie {
+
+    class Node {
+        public char val;
+        public boolean ends;
+        public HashMap<Character, Node> children;
+
+        Node() {
+            this.children = new HashMap<>();
+        }
+    }
+
+    public Node root;
+
+    public Trie() {
+        this.root = new Node();
+    }
+
+    public void insert(String word) {
+        Node curr = this.root;
+
+        for (char ch : word.toCharArray()) {
+            curr = curr.children.computeIfAbsent(ch, c -> new Node());
+            curr.val = ch;
+        }
+        curr.ends = true;
+    }
+
+    public boolean search(String word) {
+        Node curr = this.root;
+
+        for (char ch : word.toCharArray()) {
+            curr = curr.children.get(ch);
+            if (curr == null)
+                return false;
+        }
+        return curr.ends;
+    }
+
+    public boolean startsWith(String prefix) {
+        Node curr = this.root;
+
+        for (char ch : prefix.toCharArray()) {
+            curr = curr.children.get(ch);
+            if (curr == null)
+                return false;
+        }
+        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);
+ */

word-break/forest000014.java

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+/*
+
+Time Complexity: O(s * L^2)
+    - L is max length of wordDict[i]
+Space Complexity: O(s + w * L)
+*/
+class Solution {
+    public boolean wordBreak(String s, List<String> wordDict) {
+        Set<String> wordSet = new HashSet<>();
+        int maxLen = 0;
+        for (String word : wordDict) {
+            wordSet.add(word);
+            maxLen = Math.max(maxLen, word.length());
+        }
+
+        boolean[] dp = new boolean[s.length() + 1];
+        dp[0] = true;
+
+        for (int i = 0; i < s.length(); i++) {
+            for (int j = 1; j <= maxLen; j++) {
+                int beginIdx = i - j + 1;
+                if (beginIdx < 0)
+                    continue;
+                if (wordSet.contains(s.substring(beginIdx, i + 1)) && dp[beginIdx]) {
+                    dp[i + 1] = true;
+                    break;
+                }
+            }
+        }
+
+        return dp[s.length()];
+    }
+}