add: word break;

Author: Yongseok-Choi-dev

word-break/YoungSeok-Choi.java

@@ -0,0 +1,80 @@
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+// DFS 완전탐색할 때 불필요한 탐색을 줄이는 방법을 항상 고민할 것.
+class Solution {
+    public int size = 0;
+    public Map<String, Boolean> failedMap = new HashMap<>();
+    public boolean wordBreak(String s, List<String> wordDict) {
+        size = wordDict.size();
+
+        Set<Character> sSet = new HashSet<>();
+        for (char c : s.toCharArray()) {
+            sSet.add(c);
+        }   
+
+        Set<Character> wSet = new HashSet<>();
+        for (char c : String.join("", wordDict).toCharArray()) {
+            wSet.add(c);
+        }
+
+        if(sSet.size() > wSet.size()) {
+            return false;
+        }
+
+        return dfs(s, wordDict);
+    }
+
+    public boolean dfs(String s, List<String> wordDict) {
+        if(s.length() == 0) return true;
+        if(failedMap.containsKey(s)) return false;
+
+        for(int i = 0; i < size; i++) {
+            String word = wordDict.get(i);
+            if(s.startsWith(word)) {
+
+                s = s.substring(word.length());
+                boolean result = dfs(s, wordDict);
+
+                if(result) {
+                    return true;
+                } else {
+                    failedMap.put(s, true);
+                }
+
+                s = word + s;                
+            }
+        }
+
+        return false;
+    }
+}
+
+// 특정 문자로 시작되는 것만 판단하여 반복해 풀려고 했던 접근법.
+// 전체 조합을 보아야 하는 "cars", ["cars", "ca", "rs"] 경우에 반례가 됨.
+class WrongSolution {
+    public boolean wordBreak(String s, List<String> wordDict) {
+        int size = wordDict.size();
+        
+        while(true) {
+            boolean isMatched = false;
+            for(int i = 0; i < size; i++) {
+                String word = wordDict.get(i);
+                if(s.startsWith(word)) {
+                    isMatched = true;
+                    s = s.substring(word.length());
+                    break;
+                }
+            }
+
+            if(!isMatched) {
+                break;
+            }
+        }
+
+        return s.length() == 0;   
+    }
+}