[essaysir] WEEK 05 Solutions

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

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+class Solution {
+    public int maxProfit(int[] prices) {
+        int result = 0;
+        // prices = [7,1,5,3,6,4]
+        int lt = 0;
+        for ( int i = 1 ; i < prices.length; i ++){
+            if ( prices[i] < prices[lt] ){
+                lt = i;
+            }
+            else{
+                result = Math.max(result, prices[i] - prices[lt]);
+            }
+        }
+
+        return result;
+    }
+}

encode-and-decode-strings/essaysir.java

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+public class Solution {
+    /*
+     * @param strs: a list of strings
+     * @return: encodes a list of strings to a single string.
+     */
+    public String encode(List<String> strs) {
+        StringBuilder sb = new StringBuilder();
+        for (String s : strs) {
+            sb.append(s.length());
+            sb.append('#');
+            sb.append(s);
+        }
+        return sb.toString();
+    }
+
+    /*
+     * @param str: A string
+     * @return: decodes a single string to a list of strings
+     */
+    public List<String> decode(String str) {
+        List<String> res = new ArrayList<>();
+        int i = 0;
+        while (i < str.length()) {
+            // '#' 앞의 숫자를 길이로 파싱
+            int len = 0;
+            while (str.charAt(i) != '#') {
+                len = len * 10 + (str.charAt(i) - '0');
+                i++;
+            }
+            i++; // '#' 건너뛰기
+
+            // 정확히 len 글자만 잘라내기
+            res.add(str.substring(i, i + len));
+            i += len;
+        }
+        return res;
+    }
+}

group-anagrams/essaysir.java

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+class Solution {
+    public List<List<String>> groupAnagrams(String[] strs) {
+        // 아나그램을 그룹별로 만들어라
+        // 아나그램 -> 각 string 을 배치해서 만들 수 있는 가 ?
+        // 각 알파벳이 몇개가 있는 가 ?
+        List<List<String>> answer = new ArrayList<>();
+        Map<String, List<String>> map = new HashMap<>();
+
+        for ( int i = 0;  i < strs.length; i ++) {
+            char[] chars = strs[i].toCharArray();
+            Arrays.sort(chars);
+
+            String s = new String(chars);
+            map.computeIfAbsent(s, k -> new ArrayList<>()).add(strs[i]);
+        }
+
+        for (List<String> group : map.values()) {
+            answer.add(group);
+        }
+
+        return answer;
+    }
+}